Chapter 8 - Shafts And Attachments

Question 1

Why is the fit of shafts and their attachments important to machines?

Answer 1

• the fit with which parts are joined is crucial to smooth running and long life of the machinery

Question 2

What at the purposes of keys?

Answer 2

• keys and keyseats (keyways) are used to locate parts precisely
• provides a positive means of transmitting torque between two mating components
• key is removable to facilitate assembling and disassembling the shaft and components
• designed to precisely align components with mating components
• may be utilized as a safety device. If excessive torque is applied, the key shears

Question 3

What dictates the choice of a key?

Answer 3

• shaft size
• hub diameter
• style of mounting
• torque load exerted
• material type
• permanent or semi-permanent installation
• intended use
• engineering specifications

Question 4

Describe the following:

- parallel keys

Answer 4

• the top and bottom are parallel and the sides are parallel
• usually requires hand-fitting to the keyseat
• typical key-stock material is a low- to medium-carbon steel

Question 5

Describe the following:

- square and rectangular keys

Answer 5

• the most common forms of parallel keys, having square or rectangular cross sections
• rectangular keys are sometimes referred to as “flat keys”
• square keys are most often recommended for shaft diameters up to 6-1/2”
• rectangular keys are used in shafts of larger diameters (+6-1/2”)
• parallel keys are retained in keyseats, sometimes secured by set screws

Question 6

Describe the following:

- offset keys

Answer 6

• also called a “stepped key”
• adjusts for a shaft keyseat of a different width than that of the hub keyseat
• may also be used to align machine parts
• facilitate rapid disassembly and reassembly of the machine

Question 7

Describe the following:

- saddle keys

Answer 7

• shaped to fit the curved surface of the shaft
• fitted into the keyseat cut in the hub - no keyseat is required on the shaft
• several set screws are normally placed on top of the key to force it upon the shaft
• depends upon the friction between shaft and key to provide the drive when the shaft rotates - suitable only for light drive fits

Question 8

Describe the following:

- boxed (blind) keys

Answer 8

• square or rectangular key that has both its ends rounded
• “buried” and “Pratt and Whitney” keys are both types of boxed keys
• key fits into a keyseat machined into the shaft with closed-profile ends
• key length is equal to the length of the keyseat, and less than the width of the hub

Question 9

Describe the following:

- sunk keys

Answer 9

• similar in shape to square boxed keys
• sunk keys are set deeper into the shaft than plain keys
• adapted for applications where the mating machine components restrict accessibility at both ends of the key
• come in sizes from: No.1 (1/2” X 1/16”) to No.34 (3” X 5/8”)

Question 10

Describe the following:

- feather keys

Answer 10

• permit end movement of the two parts either in operation or during assembly
• come in two types: Fixed and Sliding
• Fixed feather
• • tight fit in keyseat - sometimes secured by screws
• • allows hub a certain amount of axial sliding motion on the shaft
• • prevents the rotation of one part without the other
• • sunk keys can serve as fixed feather keys
• Sliding feather
• • prevent the rotation of one part on the other
• • permit greater axial (end) movement between the parts
• • key slides with the part along the shaft in a keyway cut the length of the shaft
• • key is secured in the outer hub or part by means of a keyseat and shoulders at each end of the key
• • key can also be secured by a pin on the feather key - the pin fits into a hole drilled perpendicular into the center of the keyseat

Question 11

Describe the following:

- gib-headed keys

Answer 11

• square or rectangular key with an enlarged head on one end
• gib allows for quick extraction and is used where parts are removed at frequent intervals
• Safety - restrict the use of gib-headed keys protruding from shafts. This avoids the problem of material tangling during rotation

Question 12

Describe the following:

• tapered keys
• tapered gib-headed keys

Answer 12

• Tapered Key
• • given a 1/8” per foot taper (metric 1:100)
• • a matching taper is applied to the keyseat in the hub
• • length of key should be the same as the hub
• • can locate a component on a shaft and prevent axial movement
• • no set screw is required
• • allows for quick installation and disassembly
• Tapered Gib-headed key
• • tapered key made with a gib-head
• • same body dimensions as standard tapered keys

Question 13

Describe the following:

- woodruff keys

Answer 13

• semicircular key (full radius), also available with a flat bottom or flattened ends for keys > 1-1/2”
• circular shape permits the key to align itself to suit either a parallel or a tapered shaft
• design minimizes any tendency of the key to tip when load is applied
• permits parts to mounted or dismounted freely - used extensively for light-duty applications (small electric motors ,hand wheels, and small gears to tapered end shafts)
• ANSI code designations for purchasing:
• • last two digits give the nominal diameter (1/8”)
• • the digit(s) before the last two give the width of the key (1/32”)
• Rule of thumb
• • width of key is 1/4 the nominal diameter of shaft
• • key diameter size is the same as that of the shaft
• keys can be made from round bar stock of the required diameter and reference to dimensions

Question 14

Define keyseats

Answer 14

• grooves cut into bore and shaft
• all keyseats cut into the outside of shafts or cylinders have their sides in line with the axis of the shaft
• parallel keyseats have their bases flat and parallel to the shaft’s axis
• woodruff keyseats have a radius at the base

Question 15

Describe parallel keyseats both open and boxed

Answer 15

• open keyseats
• • cut at the end of the shaft
• • used where the exact position of the part is unknown or may vary slightly
• • used where a key is to be installed after the hub is to be located or where the key is to be removed before disassembly (ex. gib-headed keys)
• • keyseat in the shaft and hub is designed so that exactly one half of the height of the key is bearing on the side of the shaft keyseat and the other half on the hub keyseat
• • shape of the end of the keyseat determined by the type of milling cutter used to cut the groove
• – endmills - cut a profile keyseat
• – horizontal - cut a sled runner or runout keyseat
• boxed keyseats
• • cut anywhere along the shaft
• • used when the exact location of the part is known
• • length of the keyseat is generally shorter than the hub width
• • used with: square, rectangular, fixed feather or Pratt and Whitney keys

Question 16

Describe how the depth of a keyseat is measured

Answer 16

• measured diametrically from the bottom of the keyseat to the opposite side of the shaft
• measurement taken from 180* degrees from keyseat to required keyseat depth

Question 17

Describe the cutters used to cut woodruff keys

Answer 17

• for keys up to 1-1/2” diameter
• • shank-type woodruff keyseat cutters are used
• for keys larger than 1-1/2” diameter
• • arbor-mount keyseat cutters are used

Question 18

When is keyseat location important?

Answer 18

• where the strength or timing of the attachment is important, keyseat location is also important

Question 19

How deep are keyseats cut in a hub?

Answer 19

• parallel keyseats

- - cut deep enough to accommodate half the height of the key plus clearance

Question 20

Which keyseat is tapered for a tapered key?

Answer 20

• tapered keyseats

Question 21

According to the rule of thumb, what width key would be used for a 1” shaft?

Answer 21

• rule of thumb
• • the key width is nominally one quarter of the shaft diameter

= 1/4” key width

Question 22

What equipment is used to make keyseats in the field?

Answer 22

• portable keyseat cutter (115V or 230V)
• • need correct endmill for profile
• • do not overtighten the clamp. doing so may distort base and cause misalignment
• • make sure unit is grounded
• broach + arbor press (1/16” removed per pass)
• • 8-10* degree rake
• • always lubricate the back of the broach and use cutting fluid on the teeth
• • push the broach with firm steady pressure
• • make sure at least 2 broach teeth are engaged at all times
• • stop and check for alignment during each pass
• • “one-pass” keyway broaches cut 1/8” in one pass

Question 23

Describe how keys are installed

Answer 23

• parallel keys
• • fitted and assembled into the shaft’s keyseat before the hub is assembled
• • light-coating of anti-seize or oil, or small chamfer allos for easier assembly
• tapered keys
• • fit into components after assembly
• • where several components are on the same shaft, match-mark every key and component to keep sequence

Question 24

Define class of fit with regard to keys and keyseats

Answer 24

• Class 1 - clearance fit
• • free fit for key to slide in keyseat
• Class 2 - tight fit
• • slight interference with keyseat
• • hub should have tight slide fit on the key
• Class 3 - interference fit
• • for permanent assemblies
• • should be no relative movement between shaft and hub

Question 25

Describe fitting procedure for both square (or rectangular) and a tapered key

Answer 25

• Square or rectangular key fitting
  1 - accurately measure the keyseat width and depth in the shaft and hub to establish the basic standard key size required
  2 - measure the hub length to determine the key length
  3 - cut the key from key stock that is slightly longer than necessary to allow for finishing to length
  4 - assess the class of fit required
  5 - if necessary, file the key, measuring the width occasionally to control parallelism (better to file along length of key)
  6 - draw file the key to finished size
  7 - after completing each filing operation, slightly chamfer the edges of the key. Heavy chamfering reduces the area of the side, with consequent heavier loading per unit area
• Tapered key fitting
  1 - assemble the hub and check the taper by measuring at each end of the keyway
  2 - cut a piece of stock generously longer than required
  3 - fit the key to the bottom and sides of the keyseat
  4 - file the key to match the taper of the hub
  – a - mark the taper of the hub on the side of the key
  – b - file a step on each end of the key blank to a little over the marks of the taper
  – c - file the excess metal on the top of the key blank until a flat surface is formed between the two points
  5 - remove sharp corners with a file so that the key will not bind during fitting
  6 - clean the key, then smear the tapered surface lightly with prussian blue, and drive the key firmly into the keyseat
  7 - remove the key, and file the high spots which will be indicated by the bright spots or lines on the key
  8 - repeat steps 6 + 7 until the key bears evenly on top and bottom
  9 - after the taper of the key is fit, trim the key to the required length
  10 - smear the key with light oil and drive it tightly into position. If it is a gib head key, the point should be level with the end of the hib and the gib head at least 6mm (1/4”) from the front of the hub
• Always lightly coat a key with oil or anti-seize agent before driving it into the keyseat, otherwise it is liable to seize and damage both key and keyseat

Question 26

List the methods of securing keys to their keyseats

Answer 26

• some are secured by the keyseat
• pinned to hub or secured with fasteners
• using adhesives (heat can reduce strength of bond <93*C)
• set screws

Question 27

How are set screws selected?

Answer 27

• categorized by style of heads (forms) and points

- selected based on function, safety, etc.

Question 28

List the types of set screws and state their uses

Answer 28

• square-head
• • screws protrude from component (entanglement safety issue)
• flush-seating
• • hex-head screws threaded entire length
• • internal driving socket
• • most common form used in industry
• cup-point
• • used to lock pulleys, sheaves, collars, gears and parts onto soft shafts and sometimes keyed shafts
• • screw cuts into the metal of keys or shafts - giving axial and torsional holding power without an increasing installation torque
• • screws cut into shafts produce burrs which affect disassembly causing parts to “bind” up
• • “knurled cup point” selected where vibration is concerned
• flat-point
• • used to secure components: stops, screws, collars, cams, and gears to hardened shafts and keyed assemblies
• • can be disassembled without damage to shaft by screw
• cone-point
• • same application as cup-point screws
• • applied directly to shaft
• • location is scribed and shaft is then spot-drilled to engage the conical point of the screw
• oval-point
• • used to lock parts that are adjusted frequently relative to each other
• half-dog-point
• • frequently used to engage directly in slots milled longitudinally in shafts
• • point also acts as a stop to limit travel
• • allow lengthwise movement, but prevent rotation
• full-dog-point
• • same application as half-dog-point

Question 29

What determines the holding power of set screws?

Answer 29

• point type
• thread finish
• thread lubrication (ensures maximum holding power)
• diameter
• size
• location
• number of set screws

Question 30

Where are set screws located in the hub?

Answer 30

• often over the key
• placed at the hubs longitudinal centre and located over the keyseat
• if more than one set screw is needed, place the set screws 90* degrees to each other

Question 31

How are set screws locked in place?

Answer 31

• socket locking screws
• • need not be completely removed for resetting the set screw to a new position. Simply back up until the set screw socket can be reached through the locking screw hole

Question 32

When should set screws be replaced?

Answer 32

• always replace set screws when equipment is given a major overhaul or when there is any sign of wear to the head or threads
• stainless steel set screws should be replaced every time that they are serviced. Work-hardening can cause the head to fail and reduce the thrust exerted, reducing holding power

Question 33

How are gib-headed keys removed?

Answer 33

• driving a fox wedge between the gib and the face of the hub
• you may need to lubricate the face of the hub and use more than one wedge to facilitate a straight pull on the key gib
• take care to support the fox wedges so that they don’t fly loose and injure someone or damage other equipment
• if the tapered key cannot be driven out, the hub can be pushed out along the shaft to release the grip of the key

Question 34

Describe the removal of parallel keys

Answer 34

• always plan the task
• where the key is located at the end of a shaft, a hole may be drilled and tapped into the end of the key. This allows the attachment of a slide hammer to draw the key out
• other methods may be employed to grip the key, such as welding a rod to the key, or gripping it with a pair of locking pliers
• keyseats that are longer than the hub allow using a key drift to drive the key out. A “dutchman” spacer may need to be inserted into the keyseat behind the key, allowing it to be driven all the way out
• when a key cannot be removed by using these methods, it may be necessary to remove the part with pliers

Question 35

Define the following:

- shaft

Answer 35

• the component of all mechanical devices that transmits motion and power
• usually carries power-transmitting attachments such as gears, belt sheaves, or chain sprockets

Question 36

Define the following:

- axle

Answer 36

• rotating device on which a wheel is mounted
• loaded transversely
• subject to bending

Question 37

Define the following:

- spindle

Answer 37

• a slender pin or rod which turns, or on which something else turns
• usually used to directly carry a tool for doing work
• must be accurately installed

Question 38

Define the following:

- journal

Answer 38

• the part of the spindle, shaft, or axle that rotates in or on a bearing
• plain friction bearings are often referred to as journal bearings, because they come in direct contact with the journal

Question 39

What influences the selection of shafting?

Answer 39

• torque (twisting action)
• speed requirements
• power-transmitting components and their mounting methods
• compression and tensile limits
• contraction, bowing, or expansion limitations (distance between shaft supports)
• material and surface finish
• cost

Question 40

List the types of shafting available and state their uses

Answer 40

• hot-rolled shafting
• • surface has a dark, rough, oxidized finish resulting from rolling the metal while it is hot
• • “pickling” and “bright dipping” may be used to remove the surface scale, but the rough surface remains
• • not intended for direct incorporating into finished products. quality not strictly controlled
• • generally requires finishing by some machining process
• finished shafting
• • has a smooth surface finished
• • manufactured to close tolerances
• • can be incorporated into finished products
• • high cost
• • finishing methods: cold rolling (cold finished), machining, centreless grinding, grinding and polishing, and chrome plating
• AISI C1018
• • basic cold-finished steel in low-carbon range that welds readily
• • general shafting purposes, spindles, pins, etc.
• • not recommended for high speed/stress
• • low cost
• • easy to machine
• AISI C1045 / C1050
• • known as “precision” shafting, because its surface finish is precision-ground and polished
• • made from medium-carbon steels with high strength and are distortion-free
• • can be used for high-speed applications
• • also available with chrome-plated fishing for hydraulics
• • do not remove shafts from fibre tubes until after cutting
• Alloy shafting
• • where wear and corrosion resistance is necessary
• • chrome alloys (stainless steel) - hard, wear-resistant and corrosion-resistant
• • high-manganese alloys - long durability (crankshafts)
• • nickel-chrome-steel - high inherent strength (crankshafts)
• • brass / bronze - tough, wear-resistant and corrosion-resistant
• Hollow shafting
• • used for power transmissions
• easy handling
• weight loss of shaft is more than strength loss
• 4” w/ 2” bore = 25% weight loss + 6.25% strength reduction

Shafts are used to:

• transfer torque from a driver to a driven sheave, gear, pulley, or sprocket
• support pieces of equipment
• permit equipment to pivot on a point to transfer motion
• permit a driven component to slide along the shaft while transferring power
• extend the length of a drive (piston rod and a piston)
• change rotary motion to reciprocating motion (crankshafts)
• provide a support for loads applied axially
• act as guides for slides

Question 41

How is shaft material identified?

Answer 41

• proper labelling is the best way to identify shafting materials, along with proper storage and housekeeping
• surface finish, colour, weight, and magnetic properties
• results from spark tests, hammer-and-chisel tests, and file tests
• AISI/SAE designations
• colour coding is NOT a reliable means of identifying shaft material

Question 42

Define tensile and compressive strength

Answer 42

• Tensile
• • if the stress (load) tends to stretch or lengthen the shaft
• Compressive
• • if the stress tends to compress or shorten the shaft

Question 43

Describe bending stress, shear stress and torsional shear stress

Answer 43

• Bending stress
• • a combination of tensile and compressive stresses
• Shear stress
• • occurs when the applied force tends to cut through the shaft
• Torsional shear stress
• • when torque, or twisting moment, is applied to a shaft, it tends to deform by twisting, causing rotation of one part of the shaft relative to another, causing shear stress in the shaft (torsional deformation)
• • the distribution of stress is not uniform

Question 44

List the sources of shaft fatigue

Answer 44

• the weight of the shaft
• components fitted to the shaft
• loads applied to the shaft
• location of supporting bearings

Question 45

What designs are employed to reduce stress concentrations in shafts?

Answer 45

• start by maintaining the surface finish of the shaft. Protect it from nicks, scratches, corrosion, and excess binding during service, repair, or installation.
• keys are usually made with the “edges broken” (sharp corners removed). To further reduce stress concentrations, the edges may be chamfered - matching fillets must be provided in the keyseats. Do not modify runout keyseats; provide fillets whenever possible.
• when sections of shafting must be turned down for a bearing or other machine component, the reduction in diameter should be no more than ( 1.5 : 1 ). The fillet radius should be as large as possible to reduce stress concentrations

Question 46

How is the location of a bearing chosen on a shaft?

Answer 46

• place the bearing as close to the loaded components as possible
• there should be sufficient support to prevent shaft deflection from causing fatigue stress
• when anti-friction bearings are to be fitted up to a shoulder on a shaft, the bore of the inner ring is made with a radius, but it is a small radius. The fillet radius on the shaft must be smaller yet in order for the bearing to be seated properly against the shoulder. To locate the bearing correctly, a small fillet should be maintained and the bearing inner ring should bear firmly on the shoulder

Question 47

How are shafts affected by:

- alignment?

Answer 47

• two or more shafts transmitting power from one to the other must be properly aligned to each other
• the axis of the shafts must be parallel and in line, not offset
• shafts often require realignment because of settling foundations, the effects of heat, vibration, bearing wear, etc.
• precise alignment reduces wear, vibration, and fatigue loading

Question 48

How are shafts affected by:

- critical speed?

Answer 48

• as a shaft rotates, small imbalances can cause it to vibrate. For perfect running balance, the centre of gravity of the shaft must be at the shaft centre.
• as the shaft and its components rotate, the centrifugal force generated will deflect the shaft toward the heavy side
• a point is reached where there is excessive vibration, the speed at which this vibration occurs is called the “critical speed”

critical speed depends on:

• size of the load or loads carried by shaft
• length of the shaft
• diameter of the shaft
• the kind of support bearings
• usually, most machines are not set within 20% of their critical speed. Machines that must run close to their critical speed must be in precise alignment and balanced, and have very little play in the bearings

Question 49

How are shafts affected by:

- runout?

Answer 49

• radial runout
• • occurs when the shaft and the attachments are not concentric in their rotation. Some radial runout may be tolerated, but acceptable limits will be indicated
• circular runout
• • occurs because of imperfections (ovality, bumps, etc) in the cross section of the part
• axial runout
• • occurs because attachments do not rotate perpendicularly to the shaft axis
• runout can result from:
• • bent shaft
• • worn bearings
• • poor machining
• • poor assembly of components
• excessive runout can cause vibration, premature wear, and possible seizing of components such as wear rings in a centrifugal pump

Question 50

Describe how runout is checked on a shaft

Answer 50

• checked by removing the shaft and attachments from the machine and rotating them between fixed centres (such as a centring apparatus or a lathe)
• if a shaft cannot be removed from a machine:
• • a dial indicator is used to read the amount of deviation in a surface
• • the dial indicator is fixed to a magnetic base or clamp which is attached a fixed surface
• • each surface must then be checked by measuring with a micrometer to ensure it is circular
• • circular runout can then be compared to the radial runout to confirm bent shaft, worn bearings, incorrectly machined or poorly fitted components

Question 51

Describe how shafts are resurfaced and how they are straightened

Answer 51

RESURFACING

• Metalizing
• • technique of spraying a metal coating onto a metal object
• • special metal powders or wire are fed into a spray gun, where they are melted by a flame
• • they are then sprayed and deposited on the surface
• • the shaft is prepared for this coating by machining and cleaning
• Using a sleeve
• • machine the section of the shaft that is damaged and fit a sleeve over that section
• • the sleeve has an interference fit onto the shaft
• • the sleeve is re-machined after it is fitted to maintain concentricity with the rest of the shaft

STRAIGHTENING
1 - remove the bearings and other attachments from the shaft to protect them from damage and to facilitate straightening, if necessary
2 - rotate the shaft between centres and use a dial indicator to pinpoint the maximum radial runout and its location
3 - straighten the shaft in a hydraulic or arbor press, peening with a hammer or applying small amount of heat in appropriate locations

Question 52

How are the static and dynamic positions of shafts maintained?

Answer 52

• established and maintained by the bearings

Question 53

What conditions influence shaft positions?

Answer 53

• straightness
• roundness
• size or fit with the bearing
• bearing seat concentricity
• bearing seat squareness
• the radius fillets at the shoulders on the shaft
• deflection of the shaft
• the means of retaining the bearing in position
• balancing

Question 54

How does shaft size affect bearing housing?

Answer 54

• oversize diameters invite overheating or preloading

- undersize shafting may contribute to fretting of the shaft, loosening of the bearing, or excessive internal clearance

Question 55

How does shaft surface finish affect the bearing area?

Answer 55

• most anti-friction bearings require a shaft surface finish of up to 12 micron/inch. Too rough a surface finish may result in the loss of interference fit, excessive shaft wear, and fretting of the bearing seat
• babbit and bronze sleeve bearings require a finish of up to 32 micron/inch
• the shafting should be straight and free of nicks, gouges, scratches, or burrs
• imperfections in surfaces can interfere with bearing lubrication and cause localized scoring

Question 56

How is the retention of bearings on a shaft achieved?

Answer 56

• retention of bearings on a shaft is affected by the fit, by the hardness and finish of the material, and by the deflection of the shaft.
• most anti-friction bearings are mounted on a shaft with a slight interference fit, varying by type of bearing and the application
• non-ferrous shaft materials require special attention due to differences in thermal coefficients of expansion, which can vary with temperature

Question 57

What are the purposes of a hub?

Answer 57

• a machine element fitted to components such as gears, sheaves or sprockets for the purpose of enabling the component to be attached to a shaft
• typically hubs are assembled to shafts prepared with a keyseat. To install the hub:

1 - install the key into the shaft keyseat first
2 - lubricate the shaft with a non-seize or oil product
3 - align the hub keyseat with the key
4 - slide the hub into position

Question 58

List the two general types of couplings

Answer 58

• rigid

- flexible

Question 59

What drive components can create axial thrust loads on the shaft?

Answer 59

• v-belts
• chain sprockets
• spur gears

Question 60

How are gears fastened to the shaft?

Answer 60

• gears can be keyed to the shaft, then locked in place by any of the following:
• • interference fit
• • set screws
• • plain tapered bushing
• • flanged tapered bushing
• • locking collar and set screw
• • pinning
• • retaining rings
• • tapered shaft with key and lock nut or screw
• • molding polymer; cast material attaching directly to a knurled shaft
• other applications require gear to slide on the shaft, using fixed feather key or a spline

Question 61

Describe the following methods used to attach machine components to their shafts:
- compression fittings

Answer 61

• simplest form; the hub on a sprocket can slotted axially, and a clamp (bolt) drilled through the hub
• when the clamp is drawn down tight, it forces the split hub into contact with the shaft
• the pressure of the hub on the surface of the shaft permits transmission of torque
• to maintain the sprockets relative position to the shaft, a key is inserted in the connection
• maintaining concentricity of the component can be difficult with this type of connection

Question 62

Describe the following methods used to attach machine components to their shafts:
- plain tapered bearings

Answer 62

• the bushing has a small taper on its outer surface, and the hub has a matching taper in its bore
• when the bushing is pulled into a mating hub with a prescribed number of set screws, it compresses tightly to the shaft to hold the attachment true and in its proper axial position
• a key is used as a positive means of torque transmission
• • no slipping can occur between the hub and the shaft
• for correct installation and removal, refer to manufacturers specifications

Question 63

Describe the following methods used to attach machine components to their shafts:
- flanged, tapered bushings

Answer 63

• the hub of the sprocket is bored with a small taper that matches the taper of the bushing
• the lightly assembled bushing and sprocket are slid over the shaft and secured to the shaft by tightening cap screws that compress the bushing tight to the shaft
• the sprocket is easily removed from the hub by using the jackscrews in the holes tapped in the face for the sprocket
• most hubs are drilled to allow reverse mounting
• do NOT overtighten the cap screws

Question 64

Define:

- basic hole system tolerance

Answer 64

• the design of the hole is the basic size

- the allowance (excess material on a part) is on the shaft

Question 65

Define:

- basic shaft system tolerance

Answer 65

• the design size of the shaft is the basic size and allowance is left in the bore of the mating part

Question 66

Define:

- unilateral tolerance

Answer 66

• means that all tolerance for the drawing dimension is applied in one direction only

Question 67

Define:

- bilateral tolerance

Answer 67

• means that the tolerance for the drawing dimension is applied in both directions

Question 68

Define:

- positive allowance

Answer 68

• refers to the minimum clearance between mating parts

Question 69

Define:

- negative allowance

Answer 69

• refers to maximum interference

Question 70

Define:

- clearance fit

Answer 70

• allows the mating parts to maintain some clearance between them at all times
• parts with clearance fit are relatively easy to assemble and disassemble
• they may require only hand-fitting techniques

Question 71

Define:

- interference fit

Answer 71

• maintains some resistance between the mating parts at all times
• additional equipment may be required to assemble or disassemble these parts

Question 72

Describe:

- running and sliding clearance fits

Answer 72

• running fit
• • applies mainly to rotating parts such as a shaft in a friction bearing
• • the fit must not be so close that the shaft cannot turn, or so loose that the shaft floats about
• sliding fit
• • applies to parts which slide on each other, such as a dovetail slide, in this case, the weight of one part brings it in contact with the surface beneath
• • the designated clearance relates to the surface above
• • the clearance in the top surface and slight imperfections in the contact (base) surfaces allow for lubrication
• if a gib is used to adjust the clearance between the mating parts, the class of fit can be further divided into the following types of fit:
• • close sliding
• • sliding
• • precision
• • close running
• • medium running
• • free running
• • loose running

Question 73

Describe:

- locational clearance fits

Answer 73

• intended for normally stationary parts which can be freely assembled or disassembled
• various classes from snug to medium clearance fits satisfy the needs of various applications
• example – a machine screw fit into a bolting hole

Question 74

Describe:

- transitional fits

Answer 74

• when the tolerance on the mating parts partially overlaps, so that either a clearance or an interference may result
• use to locate mating parts
• example – dowel pin and mating hole used to align parts

Question 75

Describe:

- locational interference fits

Answer 75

• used in situations where accurate location and rigid alignment is of utmost importance
• because of their tightness, these fits are not intended to be used to transmit frictional loads from one part to another
• example – an anti-friction bearing pressed into a housing

Question 76

Describe:

- force or shrink fits

Answer 76

• when the tolerance of mating parts fully overlaps, causing an interference fit
• this fit requires the mating parts to be forced or shrunk together so that they act as one unit
• pressure is put on the mating parts, which deforms them when they are assembled
• if interference is excessive, the elastic limit of the material will be exceeded and the assembled parts distorted
• in extreme cases, the ultimate strength of the metal will be exceeded and the outer part will burst

Question 77

Describe the following assembly methods:

- forced fitting

Answer 77

• one part is pressed onto another
• the most accurate method of assembly is by means of a hydraulic press
• the contacting surfaces of both members must be free from grit
• the axes of the external and internal parts stay in line with each other. An assembly arbor is often used to maintain correct alignment.
• smooth mating surfaces perfectly free from surface lubricant give the best grip
• anti-seize lubricant applied before assembly helps to reduce the possibility of seizure and surface abrasion

Question 78

Describe the following assembly methods:

- shrink fitting

Answer 78

• the hub is heated to expand it sufficiently to allow it to be easily assembled on the shaft
• after assembly the hub cools and shrinks onto the shaft
• heat sources include:
• • immersion in hot water, oil baths
• • steam
• • oxygen and gas hand torches (open flame)
• • oil, gas, electric furnaces
• • electric resistance heating
• • electric induction heating
• allowances for a shrink fit are usually determined by consulting the working drawing
• the desired amount of expansion (or contraction) enables the parts to be assembled freely without the aid of force

Question 79

Describe the following assembly methods:

- expansion fitting

Answer 79

• the shaft is cooled sufficiently, the parts assembled, and allowed to expand together
• industrial refrigerator
• • control of temperature is simple and there is little likelihood of damaging the structure of the material
• • achieves temperatures to about -50C (-58F)
• liquid air
• • achieves temperatures as low as -120C (-184F)
• liquid nitrogen
• • similar to liquid air, but safer due to the absence of oxygen
• alcohol and dry ice (CO2)
• • the part is placed in a container partly filled with alcohol to which frozen carbon dioxide (CO2) is added
• • produces temperatures of -50C to -60C (-58F to -76F) without expensive equipment
• • frost formation on the parts during assembly could be a problem

Question 80

Briefly describe the types of presses available for assembly and disassembly of equipment

Answer 80

• mechanical arbor press
• • light jobs
• • good sense of pressure
• free-standing hydraulic press
• portable hydraulic press

Question 81

What safety precautions should be observed while using presses?

Answer 81

• plan the task
• understand how pieces come apart or fit together before pressing
• stay alert and consider any person working in the vicinity
• stand to the side as you work, or use a cage or other device such as a restraining blanket
• use a face shield
• observe the force (pressure gauge) being applied
• maintain alignment
• fully support the part being pressed
• keep platen-lifting cables slack and safety pins fully engaged

Question 82

Briefly describe the types of pullers and accessories available for assembly and disassembly of equipment

Answer 82

• rod pullers
• two-jaw pullers
• three-jaw pullers
• slide hammer
• bearing splitters
• shaft protectors
• step plate adapters

Question 83

How would you select the appropriate puller for the job?

Answer 83

• depends on the type of component
• identify:
• • how the part should be pulled
• • how the part can be gripped
• • the reach (length or distance for removal)
• • the spread (opening needed to grip)
• • the force required
• for a manual puller, the diameter of the adjusting screw should be at least 1/2 as large as the shaft
• for hydraulic applications, select a puller that will withstand a force exerted in tons - 7-10X the diameter of the shaft in inches

Question 84

What safety precautions should be observed while using a puller?

Answer 84

• plan the task
• make sure that you know how the parts come apart
• select a puller large enough and suited to the job
• set up the puller correctly so that it is in line and firmly grips the part
• wear protective equipment (PPE)
• restrain the work
• apply forces gradually
• maintain clean work areas