Explore a single traction motor driving an axle through a pinion and bull gear. Change tooth counts and motor speed to compare ideal gear ratio, axle RPM and torque multiplication in an axle-hung, nose-supported arrangement.
Electrical checks for actuator-powered systems
For separate electric-actuator projects, check installed current draw and duty cycle when choosing a supply. The linked actuator resources do not size or validate a railway traction supply or controller.
When comparing a cylinder to an electric actuator, convert the discussion into stroke, force, speed, duty cycle, and installed length. Matching one specification is not enough.
The electrical margin should be based on worst-case motion, not a free-running bench test. Higher load increases current and heat, and long cable runs can reduce voltage at the actuator just when the motor needs it most.
- Size the supply for startup and stall current, not just running current.
- Check wire length, voltage drop, fuse rating, and controller output limits.
- Verify duty cycle so motor heat does not become the real failure mode.
For FIRGELLI actuator systems, start with the linear actuator power-supply sizing guide, then use the actuator duty-cycle calculator when the actuator cycles repeatedly. After the electrical limits are known, compare mechanical options in the FIRGELLI linear actuator range.
Street Railway Single Motor Interactive Calculator
Enter pinion and axle bull-gear tooth counts to inspect an ideal nose-suspended single-motor axle drive. Tooth counts determine the exact speed and torque ratio; backlash is an entered-clearance comparison only.
Equation Used
- The motor pinion and axle bull gear are ideal external gears, so the axle counter-rotates relative to the pinion.
- The dashed circles are pitch circles, not tooth profiles. Their radii are proportional to the actual entered Np and Nb and remain tangent; single registration marks rotate with the exact ratio.
- The drawing uses the source-described axle suspension bearings and resilient nose support, but dimensions are schematic.
- The backlash input is a user-entered illustrative comparison. No gear-specific allowable backlash, wear state or maintenance decision is inferred.
- Efficiency, load, tooth stress, bearing load, suspension dynamics, traction, thermal limits, fatigue and service life are not predicted.
The tangent circles represent gear pitch geometry. Their moving marks show opposite rotation at the entered tooth-count ratio; actual tooth contact and wear are not simulated.
Motor Support and Axle Drive
The motor is supported at the axle through suspension bearings, with its projecting nose supported from the truck frame. Its pinion drives a larger gear attached to the axle. U.S. Patent 2,501,307 describes axle suspension bearings, resilient nose support and pinion-to-axle gearing.
The drawing shows the support arrangement schematically. The pitch-circle radii follow the entered tooth ratio and touch at the pitch point. The gears rotate in opposite directions because they form an external gear pair.
No tooth profile, contact stress or suspension response is calculated. The spring and bearings identify support locations rather than providing a load or deflection model.
Using the Gear-ratio Comparison
Use the calculator to see how a larger axle gear or smaller pinion reduces axle speed and increases ideal torque multiplication. The numerical speeds are magnitudes; the animation also shows their opposite directions.
The tool does not calculate motor torque, tractive effort, adhesion, acceleration or grade performance. Choosing or restoring a railway drive requires the specific motor, gear, wheel, suspension and control documentation; the entered ratios do not establish service suitability.
Gear Ratio and Speed
naxle = nmotor/G
Ideal torque factor = G
Np and Nb are pinion and bull-gear tooth counts. The ideal torque factor assumes a lossless gear pair; it is not a measured torque or available rail force.
The separate Backlash Error output is the distance outside an illustrative 0.010–0.020 in interval. It is zero inside that interval, 0.010−b below it and b−0.020 above it. The interval is a fixed assumption in this tool, not an allowable tolerance for a particular gear set.
Worked Ratio Example
With 15 pinion teeth, 75 bull-gear teeth and 900 motor RPM:
- Gear ratio is 75/15 = 5.
- Axle speed is 900/5 = 180 RPM in the opposite direction.
- Ideal torque multiplication is 5, before losses.
An entered backlash of 0.025 in lies 0.005 in above the tool's illustrative interval. That arithmetic does not diagnose wear or decide whether the actual gears need adjustment.
Ratio Tradeoff and Model Limits
For the same motor speed, increasing the reduction ratio lowers axle speed. In the ideal lossless comparison, torque multiplication rises by the same factor. Actual torque, power and permissible loading require information that is not entered here.
| Shown or calculated | Outside this tool |
|---|---|
| Pitch-circle speed relationship | Tooth profile, module, interference and contact stress |
| Ideal torque factor | Motor torque, efficiency, current and heat |
| Axle RPM | Road speed, wheel adhesion and braking |
| Illustrative backlash interval | Manufacturer tolerance, wear diagnosis and maintenance instructions |
Frequently Asked Questions
Does wheel speed mean vehicle speed?
No. The result is axle rotational speed in RPM. Wheel rolling diameter and slip would be needed to determine vehicle speed.
Does the torque factor give actual axle torque?
No. It is an ideal multiplication factor. Actual torque requires motor torque and drivetrain efficiency at the operating point.
Why do the circles touch without drawn teeth?
They represent tangent pitch circles with radii proportional to the entered tooth counts. The moving marks show the ideal ratio, not a tooth-contact simulation.
Is the backlash comparison a service limit?
No. The fixed 0.010–0.020 in interval is an illustrative comparison only. Use the specific gear documentation for allowable backlash.
Why does changing backlash not change the gear ratio?
Ratio depends on tooth counts. This tool keeps the backlash comparison separate and does not model impacts or lost motion during reversals.
Can I use the tool to choose a replacement traction motor?
No. It does not evaluate motor characteristics, controls, mounting loads, thermal limits or vehicle performance.
Does the nose spring animation calculate suspension loads?
No. It indicates the support arrangement; spring deflection and bearing loads are not solved.
References
- U.S. Patent 2,501,307, Suspension of axle-hung electric traction motors: axle suspension bearings, resilient nose support, and motor pinion meshing with an axle gear wheel.
- Transportation Safety Board of Canada, report R09T0151: traction-motor suspension bearings and pinion/bull-gear axle drive terminology.
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