In Hiscox’s mechanism442, differential cam throw means selecting different cam action by moving a roller transversely from one cam to another. The description allows either lever transmission or direct thrust. The illustration here uses the direct-thrust arrangement, with two selectable circular eccentric cams to make their different strokes easy to compare.
Differential Cam Throw Interactive Calculator
A follower can be shifted across a shaft to select one of two cams with different throws. Compare the strokes and select a cam to see its roller follower move. The two views show the same selection.
Equation Used
- Two circular eccentric cams rotate with the same phase.
- The follower is a translating roller whose centre lies on the shaft centreline in the end view.
- Follower contact is constrained geometrically; the spring illustrates return, without force or speed analysis.
- The difference is measured at the follower, with no rocker multiplication.
- Selection switches between two positions rather than interpolating through an unsupported intermediate surface.
This illustrates Hiscox442’s transverse follower selection. Circular eccentric profiles and a direct-thrust follower are explicit drawing assumptions, not a reconstruction of a measured engine cam.
One follower, two selectable cams
Two cams sit at different axial positions on a rotating shaft. A roller and its guide can be positioned over cam A or cam B. The shaft continues to supply rotary input; the selected cam determines the follower’s reciprocating travel.
The upper view shows the transverse selection along the shaft. The larger end view looks along the shaft at the selected cam, roller and guide. Both use the same shaft phase. The roller remains tangent to the cam at every frame.
Hiscox describes governor-driven transverse selection. This calculator exposes selection directly so the difference is visible, but it does not model a governor or a running transfer between profiles. Changeover clearance and timing would require additional design information.
Variable throw by profile selection
The historical source places this arrangement among gas-engine valve mechanisms. Its relevant principle is selecting between cam throws. It should not be confused with simply giving an engine’s intake and exhaust separate fixed lobe lifts.
The illustration uses circular eccentric profiles and a directly guided follower. The source also mentions a roller carried on a lever; that would require lever dimensions and a separate output-motion calculation.
Stroke comparison and roller contact
Let A and B be the two follower strokes. Their signed difference is A − B. Dividing that difference by A gives the relative difference; dividing B by A gives the stroke ratio. Equal strokes produce zero difference. A negative result means B has the larger stroke.
For the circular eccentric profile used here, the cam centre is offset by e from the shaft axis, and the follower stroke is2e. If the cam radius is R and roller radius is r, the distance between their centres is R+r. With cam-centre coordinates (e sinθ, −e cosθ), the roller centre above the shaft is at y = −e cosθ − √((R+r)² − e²sin²θ).
This exact contact construction keeps the roller on the surface rather than drawing an unrelated sinusoidal follower. The maximum-to-minimum roller displacement is the entered stroke. The profile proportions are illustrative and do not determine contact stress, allowable speed or spring force.
A small change in selected stroke
With A =0.345in and B =0.330in, the difference is0.015in. Relative to A this is4.35%, and B is95.65% of A. Selecting B changes the illustrated follower’s peak-to-peak travel to0.330in.
Set the strokes equal to see identical travel at either selection. Set B larger than A to reverse the sign of the difference. The comparison does not predict engine breathing, horsepower or torque.
What profile selection does not establish
Follower stroke alone does not specify an engine valve event. Cam shape, dwell, phasing, transmission geometry and valve clearances also matter. The circular eccentric example has continuous motion rather than a production valve cam’s timed rise, dwell and return.
A practical selector needs a defined transition procedure and suitable support. The displayed position switch is a comparison tool, not proof of a collision-free shift under load. Likewise, the visible spring is a return-force symbol rather than a sized component.
Differential cam throw questions
Does this compare intake and exhaust tuning?
No. It compares two selectable cam strokes for one follower, following the historical mechanism’s description.
Why are there two views?
The transverse view identifies which cam is selected. The end view makes roller contact and follower travel visible.
Can B have more stroke than A?
Yes. The signed difference becomes negative, and selecting B gives the larger travel.
Does the selector simulate a governor?
No. It chooses one of two positions. Governor motion and the transfer between lobes are outside the model.
Is the shown cam a production engine profile?
No. It is a circular eccentric with exact roller contact, chosen to demonstrate the selectable-throw principle.
Reference
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances, mechanism442, printed page123: transverse movement of a rolling disc from one cam to another, with lever or direct-thrust transmission. The circular eccentric profiles are an explicit illustrative choice.
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