Peerless-Winsmith: Back Driving and Self Locking

Engineering Information

Back driving & self locking

SELF-LOCKING SPEED REDUCERS
A worm gear is said to be self-locking, or irreversible when the gear cannot drive the worm. This condition is obtained, if the lead angle of the worm is less than the friction angle, and as a consequence the efficiency for reversed driving is zero. The friction angle for static conditions will vary with such factors as surface finish and lubrication. Based upon the generally accepted value of static coefficient of friction equal to 0.15, the friction angle would be approximately 8°. However, the friction angle decreases rapidly with the start of motion, also, vibrations from nearby sources quite often upset the static condition of a locked set of gearing a sufficient amount to reduce the friction angle to a point where motion occurs. These unpredictable factors make it advisable to resort to a brake rather than to rely on the self-locking characteristics of the gearing. A worm gear set has the following self-locking qualities "at rest’’ or "in motion’’:

CASE 1 — Self-locking of the worm and gear when the load is at rest may occur with the helix angle as great as 6°. However vibrations from an outside source, or the slightest start of the worm often upsets the static condition of a locked set of gearing a sufficient amount to start motion.

CASE 2 — Self-locking of the worm and gear when the load is in motion downward requires that the load being lowered stops after the power is shut off. Worms with a helix angle of 2° or less may be required for this service.

OVERDRIVES
In the overdriving of a reducer, the slow speed shaft is the driver, and the high speed shaft is increased in speed. For this type of service there must not be the slightest tendency of the reducer to be self-locking. All applications regarding self-locking or overdrives should be referred to our engineering department for recommendations.

AGMA SUGGESTS the following with regard to "self-locking’’: "For complete assurance of irreversibility, it is advisable to resort to the use of a brake rather than to rely on self-locking characteristics of the gearing."