Specification of Stud and Yoke Type Cam-Followers.

Commercial cam-followers are optimized for rolling along a cam-profile or cam-track.

To use them effectively, we must understand their design.

The difference between a 'Normal' Bearing and a 'Cam-Follower' Bearing?

Contact Stress evenly distributed around outer ring when in a 'Bearing Housing'.

Normal Bearings : The outer-ring is supported around its circumference within a bearing housing.

•Most usually, the outer-ring is fixed in a housing and the inner-ring rotates with a shaft.
•The contact between the outer ring and its housing is 'conformal' and with a small interference fit to make sure the bearing ring does not rotate in the housing. The load is distributed around the outer housing.
•Thus, the outer-ring does not distort when a force is applied, since it is supported by the housing (unless the housing distorts). The Hertzian contact stress is small.

The image to the left shows the bearing supported in a housing.

Contact Stress concentrated at contact when outer bearing ring in contact with a cam or track.

Cam-Follower Roller Bearings : The outer-ring is not supported around its circumference.

•The outer-ring rotates and the inner-ring is fixed.
•The contact between the outer-ring and the cam-track is only a line (or a very small patch) across the width of the cam-track and follower bearing. The Hertzian contact stress is relatively high.
•There is a tendency for the outer-ring to distort when a force is applied.

To reduce distortion, the cross-section of the outer-ring is much deeper than that of a 'normal' bearing.

The image to the left shows a cam-follower rolling on a cam that is below.

perseverance

Maximum Cam-Follower Angular Acceleration

Maximum Cam Follower Acceleration

The cam-follower's outer-ring does not rotate at constant angular velocity.

Nominally, the outer-ring's :

•Minimum angular velocity is when it is in contact with the cam's Base-Circle
•Maximum angular velocity is when it is in contact with the cam's maximum radius [nose in the case of the sketch to the left].

The cam-follower must accelerate and decelerate between the minimum and maximum velocities.

The maximum angular acceleration of the cam-follower, is a function of its friction, radius, and inertia. The friction-force is not constant because the contact-force is not constant, and lubrication is not steady.

α max = F f × R I r o l l e r

 

α max = maximum acceleration
F f = friction force between roller and cam
R = roller radius
I r o l l e r = inertia of roller

Maximum Cam-Follower Rotational Speed

^^ Nominally, the maximum Angular Velocity of the Cam-Follower Bearing is at the maximum radius of the Cam ^^

^^ Nominally, the maximum Angular Velocity of the Cam-Follower Bearing is at the maximum radius of the Cam ^^

Maximum Rotational Speed (RPM).

The maximum possible speed of a bearing is a function of its design and how it is lubricated. The maximum speed is usually given in the bearing catalogues. Alternatively, you can use this table and this simple equation:

Table Parameter = stud or mean diameter(d) × speed(RPM)

Bearing Configuration

Lubrication:

Grease

Oil

Needle Rollers with Cage

84,000

140,000

Needle Rollers as Full compliment

42,000

70,000

Cylindrical Stud Rollers / Roller Followers

66,000 / 72,000

110,000 / 120,000

For example: Maximum Speed of a Cylindrical Roller; with a internal diameter [ stud diameter] of 10mm;

Grease Lubrication: 66000÷10 = 6600RPM

Oil Lubrication: 111,000÷10 = 11000RPM

Minimum Load

Reasons to apply 'preload' to a bearing are: enhanced stiffness, reduced noise, improved shaft guidance, extended bearing service life, improved running accuracy, prevent skidding of the internal needles, rollers, or balls relative to the inner and outer rings.

The minimum radial cam-follower load, Fmin should be found as a ratio of the static load capacity, Co of the Cam-Follower Roller.

Fmin : Co

Typically, Fmin should not be less than 1 / 60 of Co

Skewing and Tilting

SKEW [view of Roller above Track]

SKEW
[view of Roller above Track]

TILT  [sectional view of Roller and Track]

TILT
[sectional view of Roller and Track]

SKEW

Rollers that 'skew' increase the axial load and axial slippage between the cam and cam-follower. The cam and cam-follower skew angle specification is typically:

•Skew Angle < 1.4 x 10-4 (°) or < 2.5 x 10-3 (m.rad).

TILT

The contact-stresses that are experienced by a 'barrel' cam-follower and a 'cylindrical' cam-follower are different.

If the cam-follower does-not tilt then the:

•maximum contact stress of a 'cylindrical' cam-follower is less [marginally] than the maximum contact-stress of a 'barrel' cam-follower.

If the cam-follower does tilt then the:

•maximum contact-stress of a' cylindrical' cam-follower is more than the maximum contact-stress of a 'barrel' cam-follower.

Thus, if you cannot guarantee alignment, you would use a barrel cam-follower.

The maximum tilt capability of a:

•Cylindrical Roller [typical] < 0.1°(1.7m.rad).
•Barrel Roller [typical] < 0.25° (4.4m.rad)

About Barrel Cam-Followers

'Barrel' cam-followers are also called 'crowned' cam-followers, and even 'spherical' cam-followers'

Other 'optimized' profiles include so called 'logarithmic' profiles.
Maximum Contact Stress of Cyclindrical and a Barrel Cam-Followers

Schematic of Maximum Contact Stress of Cylindrical and Barrel Cam-Followers

Materials and Size Tolerances.

Outside Diameter Tolerance of Bearing

Nominal outside diameter of a Cylindrical Cam-Follower Bearing is: ~h5 , nominally zero to a small under-size.

Nominal outside diameter of a Crowned Cam-Follower Bearing is: 0.00 to –0.050mm , nominally zero to a under-size.
Typical Yoke Cam-Follower Design

Typical Yoke Cam-Follower Design

INNER-RING: YES

Surface Finish

Maximum roughness should be Ra =1.6μm

Diameter Tolerance

There is a tolerance for the hole in the follower arm and also the diameter of the shaft through the inner-ring.

JS6 [J7] : Hole through follower arm

k5, g6, h6 : Stud through inner-ring, if heavy load, then k6.

Form Tolerances

Variation of Mean Shaft Diameter = 0.5 × Diameter Tolerance

Variation from Circular Form = 0.5 × Diameter Tolerance

FollowerNIR2

INNER-RING: NO

Cam-followers that do not have an inner-ring are used when space is limited.

The pin/shaft through the centre is in contact with the needle rollers. Thus, the pin/shaft should have a specification that closely replicates the inner-ring of the bearing.

Specification

Material:

Through-Hardening [ISO 683-17 eg 100Cr6]

Case hardening steels must conform to ISO 683-17 e.g. 17MnCr5, 18CrNiMo7-6) or EN 10084 e.g. 16MnCr5.

Flame and induction hardening, steels to DIN EN ISO 683-17 e.g. C56E2, 43CrMo4, or DIN 17212 e.g. Cf53.

Hardness:

•670HV

or:

•58 to 64 HRC Rockwell Hardness Scale 'C'

If the hardness of the pin/shaft is not HRC 58-64, the load capacity of the cam-follower is reduced.This table shows how dynamic and static load capacities become reduced as the shaft's hardness is reduced.

Hardness

HRC

60

58

56

54

52

50

48

45

40

35

30

25

HV

697

653

613

577

545

512

485

447

392

346

302

267

Reduction

Factor

Dyn:

1.0

1.0

.93

.84

.73

.63

.52

.43

.31

.23

.15

.11

Stat:

1.0

1.0

1.0

1.0

.96

.86

.77

.65

.50

.39

.30

.25

INNER-RING: NO. Specification... continued

Minimum Case Hardening Depth at which hardness is still greater than 550 HV, HRC 52.3:

•Depth of Hardness ≥ 0.78D  [D = Diameter of Pin]

or, always:

•Depth of Hardness > 0.3mm.

Surface Finish:

•For high speed and loads: wave free finish: Ra= 0.2μm [This also applies to shoulders and washers, if in contact with the rollers ends, or bearing rings].
•General Applications: Ra=0.35μm

Tolerance and Form: k5, k6

Variation of Mean Diameter: <0.008mm, or 0.5 × diameter tolerance, or <5μm / 25mm, whichever is the least

Deviation from Circular Form: <0.0025mm or 0.25 × diameter tolerance, or <2.5μm, for Φ<25mm, whichever is the least

High Frequency Lobing: Lobing is 10 or more times around the circumference of a shaft and if it exceeds 0.4μm from peak to valley it is called chatter. Chatter usually casues undesirable noise and reduces fatigue life.

Shaft Slope:

Slant precision <13μm/25mm

General

No nicks, burrs, scratches and dents.Oil holes are permissible in the raceway area, but care must be taken to blend the edges gently, in to the raceway, if possible put oil-hole in the unloaded zone. No grind reliefs, fillets etc

Shaft Material: Typical Steels

 

Material

JIS (Japan)

ASTM (USA)

DIN (Germany)

Through-Hardening Steel

High Carbon Chromium Bearing

SUJ2

AISI 52100

100Cr6

 

Carbon Tool

SK3 (SK2)

 

 

Case-Hardening Steel

Nickel Chromium Molybdenum

SNCM420

SNCM 8

4320

4340 [1.6565]

16CrNiMo6

17MnCr5, 16MnCr5

 

Chromium

SCr420

5120

20Cr

 

Chromium Molybdenum

SCM420

4118

25CrMo4

 

Nickel Chromium

SNC420(SNC415)

 

14NiCr10

Cam-Followers - with a Stud Shaft

CamFollowerStud-Cantilever

The 'Stud' is the shaft through the centre of the bearing. It extends to one side of the bearing so you can fix it to the cam follower lever.

Support Flange and Dimension, F

To sufficiently support the side plate of the bearing, the shoulder should be ground flat, and be larger than dimension F.

Dimension 'F' is given in OEM catalogues.

The material of the cam follower lever must support the compressive stresses of the tightening torque - see below.

Stud Diameter

A metric stud usually has a 'shaft' tolerance of h7. This is a zero to negative tolerance.

A imperial [inch] stud typically is oversized by approximately 0.025mm.

Hole in F=Cam-Follower Lever

Metric Hole tolerance : JS7 [J7]. This is a small plus and minus tolerance of the nominal hole size.

Inch Hole tolerance : F7. This is a positive tolerance on the hole.

In both cases, the stud should have a light press fit.

Stud Stress

The stress in the stud is a function of two parameters:

•Tightening Torque : a Tensile Stress, σt.

When the guidelines are followed, the tensile stress, σt , is approximately 100MPa.

•Contact Load, P, gives a Bending Stress, σb

The bending stress, is:

σb = P.L.y/I

σb = P.L./z  

Note: I, second moment of area, is reduced slightly when there is an oil-hole through the stud.

Stud fails if:

Total Stress = Bending Stress, σb, + Tightening Stress, σt, ≥ Tensile Strength, or Yield Stress, σyield

σt + σb > σyield  

Rules of thumb: the total stress on the stud should be less than:

•σ < 1370MPa when load is static
•σ < 760MPa when load is dynamic but uni-directional
•σ < 380MPa when load is bi-directional and dynamic

Thread fails if:...

...the nut becomes loose. This occurs when:

σt - σb < 0

If the compression stress due to bending exceeds the tension stress due to the nut's tightening, the nut might become loose.

To prevent the nut becoming loose, follow the tightening torque guidelines!

Hexagonal Nuts Tightening Torques [EN ISO 4032].

M6x1[3Nm]; M8x1.25[8Nm]; M10x1[15Nm]; M12x1.5[22Nm]; M16x1.5[58Nm]; M18x1.5[87Nm]; M20x1.5[120Nm]; M24x1.5[220Nm]; M30x1.5[450Nm]

Cam-Followers - General Installation Requirements

•Install the oil plug, with a small mandrel [if you are not going to re-lube it] to prevent the rolling elements being contaminated by the ingress of dirt.
•Insert a pin into the lubrication hole that is transverse to the shaft to prevent the stud rotating.
•Locate the lubrication hole that is transverse to the shaft in the unloaded direction - so it is in nominally compression.

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