TORQ ARM THEORY

Torque arm theory

Introduction

  Shaft mounted gearboxes provide one solution to meet the needs for an application, this solution is a hollow shaft gearbox mounted directly onto the equipment driven shaft which must support the gearbox. The gearbox hollow shaft can be with keyway, bushing or shrink disk. The motor may be fixed to the gearbox using an input flange adapter with coupling or fixed in the gear housing by a motor mount which the power is transmited to gearbox by pullyes and V-belt.

           Considering that the gearbox is mounted directly onto an equipment driven shaft with no other external support, it will be necessary a torque arm attached to it. The torque arm is a connecting link between the reducer and a fixed point intended to resist the torque developed by the gearbox, transmitting the reaction torque produced by the gearbox, and preventing the counter-rotation of the gearbox during operation.

 The advantage of a shaft mounted hollow shaft gear boxes is less number of components, low speed coupling is not required, compact drive arrangement, simple alignment etc., and on input side, flanged motor can reduce cost, components and space requirement.

The disadvantages of hollow shaft is oxidation of shaft which makes it difficult disassembling the gearbox after some years have past; and restricted space considering that the gearbox for disassembling must be removed over the extended shaft length engaging the width of the reducer plus pressure connection. To overcome the problem of corrosion and restricted space you can use a shaft mounted gearbox with output solid shaft mounted direct onto the equipment shaft by a rigid coupling according to DIN 116.

 Types of torque arm

 Germotors Vertimax Series

The Vertimax V-series has a built-in torque arm at the top of the gear housing (lug). Two rubber bushings, a washer, a bolt and two nuts are supplied as a standard. The following pictures show us the correct methods for installing a torque arm.

The gear housing support lug could be padded on both sides with a rubber bushing which protects the reducer in either the clockwise or counter-clockwise direction so that the gearbox is not rigidly fixed. A rigid fixation will cause stress on the bearings and gearbox shaft and the bearings may fail prematurely.

Pay attention to the direction of rotation of the hollow shaft, the urelast spring in the rubber buffer set are to be loaded under compression in the main working direction of rotation and it is recommended pre loading 3 mm per urelast spring.

For Vertimax series there is available the torque arm with turnbuckle (please see picture bellow) and in this case it is importante to pay attention to the force “F” which must to act in pure tension on the torque arm. This fact is especially important for torque arms that are long in length because, if they are exposed to compressive forces, they may buckle.The following picture shows the correct method of mounting a torque arm so that it is in tension:

This torque arm is supplied with a turnbuckle arm, which is a device commonly used to tighten a rod or rope. Its components include a sleeve with threads connections of opposite hands (left and right) at each end. Turning the sleeve, the threads will be drawn together creating tension.

Keep in mind that is importante to calculate the force (F) in order to assure that this force will not exceed the capacity of the torque arm material. This force is calculated dividing the output torque (T) by the distance from the center of the output shaft to the pivot point (L).

Planetary gearbox

  Usually you have two configuration of torque arm for planetary gearbox that is single-sided or double-sided torque arm (please see pictures bellow). For gearboxes equipped with a double-sided torque arm, the torque will be supported at the adjoining structure via supporting bearings. This design means that the machine bearings are almost totally free of transverse forces apart from the weight.   Depends on the size of the planetary gearboxes it is recommended only the option double-sided torque arm.

Parallel and Right angle helical gearbox.

 Right Angle Gearbox with motor, high speed coupling mounted to its own floating foundation.

This Picture show the total mass (WT), its center of gravity (CG) . the reaction at the low speed shaft (P), the reaction at the torque arm (TR) and the output torque (M). The direction o the torque M is quite importante, here it is assumed to be clockwise. To calculate the torque arm reaction TR, it is recommended that you sums the moments about the low speed shaft equates them to zero and them solve to TR.

TR= (M+ TotalCG x WT) / T

The design of the torq arm is importante for the reability of the gearbox. The recommended design is “d” in the picture bellow, although it remain rigid in the vertical axis to bear the load, the flexibility is keep in the other five degre of freedom. This is importante because even the gearbox is manufacture accurately there will be some relative motion between the drive and the surrounding structure, i.e. the motor will wobble slightly when operating. The torq arm design “d” allows small motion to take place unrestrained, emiminating any unexpected restraining load. If the drive assembly motion becomes restrained, an indeterminate load will be created and may cause premature failure.

Resultant Force Calculation

For smaller flexion moment on the equipment shaft the torq arm must be mounted the same side of the equipment shaft. Depend on the length of the torque arm, it can bear tension stress as well as thrust loads, and in a long torque arm, bearing thrust load and the compression load must be considered, avoiding buckling of the torque arm.