Selection in Practice

Checklist: From Requirements Profile to Gear Type

A gearbox is never selected based on a single criterion. Eight questions from the previous units together form the requirements profile:

• Torque – Rated Torque and Peak Torque at the Output
• Speed – Input and Output Speed, and the Resulting Gear Ratio (Unit 1)
• Axis alignment—parallel, coaxial, or perpendicular (Unit 3)
• Duty cycle – operating hours per day; in continuous operation, efficiency is a more significant factor (Unit 2)
• Positioning accuracy—is low torsional backlash required? This tends to favor planetary gears over worm gearboxes.
• Installation orientation – horizontal or vertical output shaft
• Environment – Temperature Range, Lubrication Concept
• Budget – Capital vs. Operating Costs Over the Service Life

If the answer to “duty cycle” is “continuous operation, long runtime,” the choice shifts almost automatically away from the inexpensive but less-efficient worm gearbox (Unit 2)—unless self-locking is required for safety reasons at the same time (Unit 4).

How to Read a Data Sheet Correctly

Three specifications in the data sheet often determine whether a design succeeds or fails:

• Rated torque – the torque for which the gearbox is designed for continuous operation; peak torques (start-up, shock) must be checked separately.
• Application factor K_A – multiplies the actual load torque to account for shock loads and duty type; according to ISO 6336, for shock-loaded drives (conveyors, presses), it is typically 1.5–1.75. In the gearbox catalog, it is often referred to as the service factor. Do not confuse this with the overload factor for intermittent duty from Module 1, by which the value is divided.
• Transmission series and quality grade—the available transmission grades within a series, as well as the manufacturing quality in accordance with DIN 3961/3962, which helps determine noise level, smoothness of operation, and price.

Common Errors in Real-World Applications

These five mistakes come up time and again in TEA consulting:

1. Calculate using the rated torque and disregard the application factor K_A for shock-loaded drives.
2. Do not rely on the self-locking capability of a worm gearbox as a full-fledged brake substitute without a valid, manufacturer-certified proof of performance. DGUV Regulation 54 lists self-locking drives as one way to meet the requirement for a self-acting braking device; for winches in accordance with EN 14492-1, their braking requirements also apply (self-engaging brakes for lifting and lowering). It must be demonstrated that the self-locking mechanism will securely hold the load during operation (Unit 4).
3. Selecting a lubricating oil viscosity that is generally too high, rather than tailoring it to the speed and temperature ranges.
4. For helical gearing, do not account for the resulting axial forces in the bearing design (design the bearing to accommodate the axial force, e.g., angular contact ball bearings or tapered roller bearings, depending on the magnitude of the axial force; for calculating axial and radial forces, see module 4).
5. Omit the quality grade according to DIN 3961/3962 from the inquiry—otherwise, each supplier will assume a different tolerance, resulting in noticeable differences in noise and price.

Example: Checklist applied

A conveyor belt drive runs 16 hours a day (high duty cycle); the shafts are parallel; positioning accuracy is not required; and self-locking is not needed. The long operating time makes efficiency the most important criterion—the checklist therefore recommends a helical gearbox in standard quality, designed with an application factor of K_A corresponding to the shock load caused by the conveyed material.