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Entry type: FAQ Entry ID: 34124002, Entry date: 01/27/2009

Correct belt tension, cylindrical-roller bearings, and fractured shafts on belt drives

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How can the belt tension be set correctly, why are cylindrical-roller bearings preferred on belt drives, and why do belt drives suffer from fractured shafts?







Correct belt tension






  • The most suitable belt tension is the lowest tension at which the belt ceases to slip at full power.

  • Where there is a direct connection to the mains, the motor's breakdown torque must be exceeded during power-up.
    If this causes temporary belt slippage which ceases to be evident at the rated load, it can be assumed that the belt tension has been set at an appropriate level.
    The belt tension must never be increased to counteract belt slippage during power-up.

  • New belts should be "warmed up" for approximately 15 minutes before being run at rated power for the first time. Depending on the material used for the belt, it may stretch permanently during this period. The belt should only be tensioned at full load after this process, and only until it ceases to slip.
    The belt tension should be checked on several occasions during the first day and at regular intervals thereafter with corrections being made as appropriate.

  • Belts are subject to wear; V-belts must always be replaced as a whole set.

  • Attention must be paid to differences in performance between new and used belts.

  • Manufacturer documentation must be complied with during belt maintenance.

  • Belt tension can be checked using conventional, mechanical tension checkers or using advanced tension checkers that are capable of evaluating the resonant frequency of the belt under tension. Compliance with manufacturer documentation is essential when using such devices.

    Commercially available measuring instruments for checking belt tension

Conventional, mechanical tools for pre-tension checker

Sonic pre-tension checker


Cylindrical-roller bearings on belt drives


Belt drives generate high radial loads.

These high radial forces are often too much for ball bearings, which can sometimes fail prematurely.

In view of this, cylindrical-roller bearings are the preferred bearing elements on the DE motor side for belt drives, and their use is often a compulsory requirement in the case of more powerful motors. Clamping bars enable stepless lateral motor adjustment, whereby clamping screws are used to generate the required belt tension.



    Clamping bars with clamping screws
    Motor damage as a result of increased transverse forces

As the degree of belt pre-tension cannot be measured directly, this creates considerable uncertainty in terms of the bearings and the motor shaft load.

For fast-running motors, the permissible level of transverse force is mainly determined by the amount of fatigue suffered by the material used for the DE bearing. For slow-running motors, the permissible bending tension for the shaft is also a factor.


If the motor shaft's permissible flexural fatigue load is exceeded, tearing caused by material fatigue often results with the subsequent force causing the clearance area to fracture.

These types of shaft fractures follow a common pattern and generally occur at the shaft shoulder or below the DE bearing.


Changing the material used for the shaft or refining the transition radii to reduce the notch effects produced by the edges are suitable measures from a purely physical perspective, but these often fail to address the problem over the long term.


In most cases, the desired level of reliability can only be achieved by making changes to the design of the belt drive and/or the motor. These include using a larger belt pulley or a clamping washer, placing the belt pulley closer to the motor bearing, or increasing the shaft diameter.


a)           b)

a) Typical shaft fracture pattern at high transverse forces

    b) In comparison to shaft fracture pattern as a result of torsional
    overload with no
effect from excess transverse forces
    Increased transverse forces can also cause impermissible levels of motor shaft bending within the air gap. Sagging may be so severe that the rotor is brought into contact with the stator. This usually results in the motor being damaged beyond repair.

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