An inherent factor in the selection wormgear boxes is the efficiency η defined as the ratio between the mechanical power coming out from the output shaft, and the power in the input shaft:
Some reasons concurring to a reduction of the efficiency can be identified in the several forms of sliding and rolling friction.

In practice, efficiency depends essentially by:
· helix angle
· material of matching parts
· tooth form accuracy
· gear finishing
· lubrication
· gear sliding speed
· load vibrations
· temperature
In the combined BOX units (BOX+BOX), the total efficiency value is the result of the product
of the efficiency of the two single boxes composing the combined unit.

Dynamic efficiency ηd
It is the efficiency value that comes out after completion of the running in time of a few hours and that keeps almost constant in the subsequent time of work.
The graph no.1 shows indicatively the time required to reach the maximum value of dynamic efficiency

Static efficiency ηs
It is the efficiency obtained at start-up, particularly important in the choice of a BOX unit on those applications (like liftings) where due the very restricted time of work for each operation, the standard operating conditions are reached seldom.
In these applications it is necessary to increase properly the motor power, in order to compensate the poor efficiency of the BOX unit while starting up ηs< ηd


Some BOX units permit to lock and hold in place a load when electric power switches off. This characteristic, called irreversibility, is inversely proportional to the efficiency and the helix inclination, and directly proportional to the reduction ratio.

The efficiency of the toothing profiles is the main factor in effecting successfully the whole efficiency of the wormgear units, and it is on a large extent tied to the helix angle of profiles.

In order to get the fittest solution for a certain application, it is necessary to analyse the difference between static and dynamic irreversibility.


Static irreversibility
A BOX unit has a low static reversibility whenever it is possible to put it in rotation only through driving the output shaft with a very high torque and/or vibration or twisting of the output load.
The static irreversibility is inversely proportional to the static efficiency. Theoretically:
ηs < 50% static irreversibility
50% < ηs < 55% low static reversibility
ηs ≥ 55% good static reversibility

Dynamic irreversibility
This is the most difficult condition to get.
. It occurs whenever, at the stop of the conditions keeping the worm shaft in rotation, even the motion of the output shaft stops immediately. The dynamic irreversibility is inversely proportional to the dynamic efficiency. Theoretically:
ηd < 40% total dynamic irreversibility
40% < ηd < 50% good dynamic irreversibility
50% < ηd < 60% low dynamic reversibility
ηd ≥ 60% good dynamic reversibility
The following scheme proposes an indicative analysis of the different degrees of irreversibility based on the helix angle.

Note: Whenever a total irreversibility of a BOX unit is important for safety reasons, we strongly recommend the use of brake motors of the series Delphi ATAC or ATDC.