Evans friction cones use two equal, oppositely oriented cones on parallel axes and an endless movable leather belt in the form of a ring running between them. Cone B is urged toward cone A to squeeze the ring. This is not a conventional belt loop wrapped around both shafts.
Evans Friction Cones Interactive Calculator
Shift the endless leather ring along the opposed cones and compare the two mean contact diameters and ideal mean-line speed ratio.
Equation Used
- Equal cones with opposite linear taper.
- Mean-line contact diameters used for the displayed ideal ratio.
- Cone B is urged toward A to maintain ring pressure.
- Slip, squeeze force, torque capacity, heat and wear are not calculated.
The ratio is the ideal mean-line diameter ratio. Elements of Mechanism figure 33 explicitly notes that the ratio varies from edge to edge of the finite-width ring belt, so some slip is inherent. No torque capacity, pressure, heat or wear is predicted.
Moving the ring changes the contact diameters and the ideal speed ratio. Actual speed also depends on slip at the two contacts.
Ring belt between pressed cones
Figure 33 shows two equal cones A and B on parallel axes with opposite taper. Endless leather ring C runs between their facing surfaces. Springs or another loading method urge axis B toward A so the ring is squeezed at both contacts.
Moving C along the cone axes changes the two contact diameters in opposite directions and varies the approximate angular-velocity ratio.
Continuously adjustable speed ratio
Shifting the ring along the cones gives a continuous change in the mean contact-diameter ratio. Use the calculator to compare positions and end diameters. It does not size a drive for a particular machine or load.
Mean-line ratio
Ignoring slip, tangential speeds at the ring are equal, so nB/nA is approximately DA/DB using mean contact diameters. With equal opposite linear tapers, DA grows while DB shrinks as the ring moves.
The source warns that diameter - and therefore ideal ratio - differs from one belt edge to the other. The mean-line result cannot be treated as an exact no-slip ratio.
Mid-position example
With relative end diameters 2 and 12, the mid-position gives equal mean contact diameters of 7 and an ideal ratio of 1. At 25% position, the mean diameters are 4.5 and 9.5, for an ideal ratio of about 0.474.
Scope and limitations
Moving the ring provides continuous ratio adjustment, but finite belt width makes edge velocities disagree and produces slip. This calculator does not determine squeeze force, transmitted torque, efficiency, belt heating, wear, safe speed or bearing load.
Why actual speed can differ
The finite-width ring touches different cone diameters across its width, so its edges cannot both roll without slip. Contact condition, loading pressure and wear also affect traction. The calculator does not predict these losses or set allowable operating limits.
Comparing the calculation with a drive
The displayed ratio uses mean contact diameters and neglects slip. A measured input-to-output speed ratio may differ under load. Assess the actual ring, cone surfaces, bearings and loading arrangement before using the result for equipment selection; the historical diagram gives no universal runout, slip or temperature limits.
Questions
Is C a normal belt around both shafts?
No. It is an endless leather ring running between the two cone surfaces.
Why is the calculated ratio not exact?
The ring has finite width, so its two edges contact different diameters and cannot both roll without slip.
What keeps contact pressure on the ring?
The source states that cone B is urged toward cone A by springs or another loading method.
Primary reference
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
