Commercial cam-rollers are optimized for rolling along a cam-profile or cam-track.
To use them effectively, we must understand their design.
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.
Cam-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.
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 (its 'nose' in the case of the sketch to the left).
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.
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)
E.g.: The maximum rotational speed of a Cam-Roller bearing; with an internal diameter ( stud diameter) of 10mm;
Grease Lubrication: 66000÷10 = 6600RPM
Oil Lubrication: 111,000÷10 = 11000RPM
Reasons to apply 'pre-load' 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
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).
The maximum tilt capability of a:
•Cylindrical Roller (typical) < 0.1°(1.7m.rad).
•Barrel Roller (typical) < 0.25° (4.4m.rad)
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.
Schematic of Maximum Contact Stress of Cylindrical and Barrel Cam-Followers
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'.
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:
•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.
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
Maximum roughness should be Ra =1.6μm
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.
Variation of Mean Shaft Diameter = 0.5 × Diameter Tolerance
Variation from Circular Form = 0.5 × Diameter Tolerance
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.
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.
•58 to 64HRC (Rockwell Hardness Scale 'C')
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:
•Depth of Hardness ≥ 0.78D (D = Diameter of Pin)
•Depth of Hardness > 0.3mm.
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
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 center 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.
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.
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/J
σb = P.L./z
Note: J, the second moment of area, is reduced when there is an oil-hole through the center 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:
•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)
•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.