Two cams on one rotating shaft act on rocker levers to drive attached rods. Their relative phase sets the timing between the outputs. The separated end views below make both cam contacts and guided rods visible.
Cams on Shaft to Alternating Rod Motion Interactive Calculator
Vary tangential key clearance, key contact radius, shaft speed and nominal phase. Compare the resulting angular allowance, timing shift and two connected rod motions.
Equation Used
- Tangential clearance s is small relative to key contact radius r, using δ = s/r.
- Entire entered clearance is applied as a one-sided phase lag of cam B. No reversal, impact or elastic torsion is modeled.
- Illustrative cam body radius 28 mm, eccentricity 6 mm, roller radius 5 mm, lever 100 mm and connecting rod 60 mm.
- Both end views share shaft speed; they represent separate axial planes, not independently driven shafts.
- Rigid links, frictionless guides and maintained cam contact are assumed. Animation runs 20 times slower than entered speed.
Two end views show cams at different axial positions on the same shaft. Each cam drives a rocker and guided rod. The entered tangential key clearance produces a one-sided angular allowance at the key’s contact radius, allowing comparison of rod timing at the selected shaft speed.
One shaft coordinates two rods
Brown’s movement 149 shows rotating cams acting on levers, which move attached rods. A common shaft gives a fixed timing relationship, while the cam profiles determine each output’s movement.
The animation separates the two cam planes into end views so neither lever or roller is hidden. Each cam drives a fixed-length rocker and a connecting rod attached to a vertical guide. The illustrated circular eccentric cams are a clear geometric example, not measured dimensions from the historical drawing.
Comparing timed outputs
Several cam-driven outputs can share one rotating input. Their relative phase can be selected so movements occur together, approximately opposite one another, or at another chosen interval.
This page explores that timing relationship and a small-clearance allowance. It does not establish the timing or tolerances of a particular production machine.
Converting key clearance into timing
If tangential clearance s acts at radius r, the small angular allowance is approximately δ = s/r radians. The radius must refer to the key’s contact location. The outside cam radius is not the appropriate dimension for this calculation.
Convert δ to degrees by multiplying by 180/π. One shaft revolution takes 60000/N milliseconds at N rpm. Multiplying that period by the angular fraction δ/(2π) gives the timing allowance in milliseconds.
The displayed example applies the full allowance as a lag of cam B: effective phase equals nominal phase minus angular allowance. This is a simple one-sided timing estimate; it does not simulate how a loose connection changes contact sides when torque reverses.
A small phase allowance
With 0.10 mm tangential clearance at a 30 mm key contact radius, δ is 0.003333 radians, or approximately 0.191°. At 250 rpm, one revolution takes 240 ms and the corresponding timing shift is about 0.127 ms.
A nominal 180° setting therefore becomes 179.809° in the illustrated one-sided case. Doubling shaft speed halves the time error while leaving the angular allowance unchanged. Increasing the contact radius reduces the angle produced by the same clearance.
Timing estimate versus complete machine analysis
The illustration shows how changing relative cam phase changes rod timing. Maintained contact and fixed linkage lengths constrain the motion through the full revolution.
It does not size the shaft or key, determine follower loading, model valve overlap, or predict collision between real machine outputs. Those require dimensions, loads and clearances beyond the timing allowance entered here.
Questions about the two-cam motion
Why are there two separate end views?
The cams occupy different axial positions on one shaft. Separate views let you see both contacts and rocker levers clearly.
Which radius should I enter?
Enter the radius where the tangential key clearance acts. Do not substitute the cam’s outer radius.
Does zero clearance guarantee perfect timing?
No. It removes only the clearance allowance in this calculation. Manufacturing phase error, elastic twist, wear and other effects are not modeled.
Why can effective phase be negative?
The one-sided lag is subtracted from nominal phase. A negative angle wraps around the shaft revolution; for example, −1° has the same orientation as 359°.
Primary mechanism reference
Henry T. Brown, Five Hundred and Seven Mechanical Movements, movement 149, pages 40–41, Cornell University Library scan. The illustration and description show rotating cams driving levers and attached rods. The clearance calculation is a separate small-angle geometric estimate.
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