Cam-Follower: Design Specifications

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

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

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

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.

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

Nominally, the outer-ring's :

The cam-follower must accelerate and decelerate as the radius of the cam changes.

However, the maximum possible 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.

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)

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

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

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:

TILT

The maximum tilt capability of a:

Barrel Cam-Followers:

Barrel cam-followers are not cylindrical. The have radius that can be seen when viewed across its section. They are intended to be used when the rotational axis of the cam-follower cannot be parallel to the rotational axis of the cam.

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

Other profiles include logarithmic profiles. Many profiles are commercially sensitive.

A Cylindrical-Follower or a Barrel-Follower?

Whether to use a Cylindrical-Cam-Follower or a Barrel Cam-Follower is a function of the type of machine and how well the follower and cam's rotational-axes can be aligned.

Cylindrical Roller - miss-alignment

When the rotational axis of a cylindrical-roller is not parallel to the rotational-axis of a cam, the cam-follower will tilt relative the cam's surface, and roll along its edge. - see the bottom and left example in the image to the left.

The contact is distorted. The maximum value of Contact-Stress is significantly greater than the nominal value that is calculated for 'line-contact'.

Barrel Rollers.

If the rotational-axis of the cam-follower roller is not parallel to that of the cam, the cam-follower will tilt relative to the cam's surface.

In this case, the contact moves across the cam surface, but when the tilt-angle is within limits, it does not roll along its edge.

The contact is not distorted much. The maximum value of Contact-Stress is not more than the nominal value that is calculated for 'elliptical-contact'.

Nominal Maximum Contact-Stress

The nominal maximum contact-stress of a barrel-roller is greater than that of a cylindrical-roller.  However, when a cam-follower tilts the actual maximum contact-stress of a cylindrical-roller is much greater than that of a crown-roller.

The permissible tilt angle of a cylindrical-roller is very small, ( <0.1° ), and thus a barrel-roller is a good design option in many cases.

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:

If the cam-follower does tilt then the:

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

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.

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

INNER-RING: NO

When space is limited, it is possible to use a cam-follower that does not have an inner-ring.

The bearing's needle-rollers are in direct contact with the pin/shaft. Thus, the pin/shaft should have a specification that closely replicates the inner-ring of the bearing.

Specification

Material:

Through-Hardening steel : ISO 683-17 : 100Cr6

Case hardening steel : ISO 683-17 : 17MnCr5, 18CrNiMo7-6; or EN 10084 : 16MnCr5.

Flame and induction hardening steel : DIN EN ISO 683-17 : C56E2, 43CrMo4, or DIN 17212 : Cf53.

Hardness:

or:

If the hardness of the pin/shaft is less than 58HRC, 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.

INNER-RING: NO... continued

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

or, always:

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: wave free finish Ra=0.35μm

Tolerance and Form: k5, k6

Variation of Mean Diameter: <0.008mm, or 0.5×diameter tolerance, or <5μm/Φ25mm, whichever is the least

Deviation from Circular Form: <0.0025mm or 0.25×diameter tolerance, or  <2.5μm/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

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:

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

The bending stress, is:

σb = P.L.y/J

σb = P.L./z  

Note: J, the second moment of area, is reduced when there is an oil-hole through the centre of 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:

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]