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The “BM” MOTOR BASES are used for the automatic tensioning of the belts in transmissions, driven by an electric motor.

As is known, belts perform the task of transmitting power if the design conditions are respected, but mainly the winding angle on the pulleys and the pull on the belt must be guaranteed. In a belt drive it is very important to have the correct tension value over time, so that the drive can always work at maximum efficiency, avoiding slipping on the driving pulley which can cause variations in the speed of the belts and the number of revolutions of the pulley conducted with consequent reduction of the available power but mainly the wear of the belts themselves and therefore their breakage. The motor mounts are widely used in the motorisation of screens, crushers and sieving machines, where the activation of an eccentric for the movement of the vibrating box causes considerable vibrations and continuous changes of center distance between the driving pulley and the driven one. Therefore, the use of an engine support which allows to absorb vibrations and to always keep the tension on the belts constant is inevitable. Once pre-loaded, the “BM” MOTOR BASES develop a twisting moment when put into operation which allows the belts to be kept automatically tensioned at all times.

The “BM” MOTOR BASES are elastic supports with a plate on which to house the electric motor. The plate is integral with the elastic group, which generates and maintains the reaction torque when pulling the belts; this is housed within a system of two square steel profiles, rotated 45° from each other and four natural rubber cylinders inserted into the resulting triangular compartments. The rotation of one profile with respect to the other elastically compresses the rubbers which supply the force to keep the motor constantly pushed on the belts.

The “BM” MOTOR BASE is activated with the preload phase, i.e. with a mechanical system by means of screws or jacks which allows the two square sections to rotate between them. The twisting moment created by the rubber, as has already been explained, is responsible for maintaining tension on the belts. The preload takes place by means of traction screws, this is an effective system, but it forces the operator to be on the machine.

Example: Motor Base 70


A) Loose belt:

With B bolts loose, You have to incline the motor support by the A screw.

This position of the motor will help You in the installation of the belts on the pulleys.

B) Tensioning stages:

1) With B bolts loose, You have to start the preloading phase by the A screw.

2) With B bolts loose, You have to go on with the preloading by the A screw until the inner elastic element will turn at the warned position (lower than 30° of rotation).

3) You have to fix the B bolts and remove the prealoding A screw before starting of the motor.

Preloading system for elastic motor bases:

A: Preloading bolt

B: Sliding block

C: Regulation holes

D: Positioning of the inner square of the elastic element

To preload the motor support (70 and 90 sizes) You have to place the sliding block (B) on the regulation hole (C) that is more suitable to the geometry of the transmission, then turning the preloading bolt (A), the plate lets the inner square (D) of the elastic elements rotate.

Cardanic supporting device:

A: Cardanic support

B: Elastic element

The 70 and 90 motor supports have, on the side on where the pulley, a reinforcement between the inner square and the external tube of the elastic element with the purpose to hold up the tension of the belts, especially by the start up, in presence of an high static torque. This support, therefore, has the feature to hold up the cardanic moment that tends to a disalignement of the inner and external square of the elastic element. For this reason in the assembly operations of the motor support You will have especially to take attention that the cardanic support will be positioned on the same side of the driving pulley.



Belt tensioning

Belt tension measuring instruments:

Q1: Initial operation test force given by guidelines of belt manufacturer

Q0: Operational test-force

In order to obtain the ideal tension must be applied Q1 force in the middle of each section of the belt, verifying that the belt deflection is 16mm with trasnmission interaxle distance of 1000mm. The relevant deflection by shorter or longer centre distance has to be interpolated accordingly. After the first running in period, the belts lose resistance, therefore the operational test-force Q0 decreased around 20% compared Q1.

Usual positioning of the Motorbase in screen drive applications

Base plate mounted in the middle over the elastic element. Plate position horizontally on base. Installation of the entire base 45° inclined (aligned to exciter).

Base plate mounted in the middle over the elastic element. Plate position horizontally on base. Motor shaft min 15° above or below the driven eccentric shaft.


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