22-06-2013, 03:21 PM
TWIN WORMGEAR SELF LOCKING SYSTEM
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PROBLEM DEFINITION
The term self locking as applied to gear systems denotes a drive which gives the input gear the freedom to rotate the output gear in either directions but the output gear locks with input when an outside torque attempts to rotate the output in either direction. This characteristics is often sought after by designers who want to be sure that the loads on the output side of te system cannot affect the position of the gears. Worm gears are one of the few gear systems that can be made self locking, but at the expense of efficiency, they seldom exceed 40% efficiency, when made self locking.
THE SOLUTION
This innovation in gearing combines two worm screws to give self locking characteristics, or to operate as fast acting brake when power is shut off.
The Twin worm self-locking system is a simple dual worm system that not only provided self locking wit over 90% efficiency, but also exhibited a new phenomenon called deceleration locking.
The twin worm gear drive is quite simply constructed , two threaded rods, or worm screws are meshed together . Each worm is wound in a different direction and has an different pitch angle. For proper mesh the worm axes are not parallel but slightly skewed. (if both worms had the same pitch angle, a normal ,reversible drive would have resulted similar to helical gears.) But by selecting proper and different pitch angles, the drive will exhibit self locking or combination of self locking and deceleration locking as desired.
CONSTRUCTION & WORKING
The Twin worm gear self locking systems consists of the following parts
1.Right hand worm gear:
The right hand worm gear is the input gear of the system , it has worm threads of 2 module which are machined with right hand of operation with helix angle of 2 degrees. The root clearance is appropriately provided , as well as the pressure angle is maintained at 14.5 degree .It is mounted at an angle of 1.5 degrees to the horizontal, this angle is the opening angle between the two worm shafts .The shaft is held between two medium duty ball bearings 6205zz ie, single row deep groove ball bearings with metallic seals at either end.
2. .Left hand worm gear:
The left hand worm gear is the output gear of the system , it has worm threads of 2 module which are machined with left hand of operation with helix angle of 5 degrees. The root clearance is appropriately provided , as well as the pressure angle is maintained at 14.5 degrees . The shaft is held between two medium duty ball bearings 6204zz ie, single row deep groove ball bearings with metallic seals at either end and it is placed parallel to the horizontal.
3.LH/RH bearing housings :
These are column members of the system that support the above two shafts in ball bearings, the bearings on the output shaft are appropriately locked using bearing caps(not shown).
4.Base plate :
The base plate is the base member that houses the entire assembly of system. The housings are bolted to the base plate.
WORKING
The twin worm gear self locking system input shaft is connected to the drive motor , which provides the input power. The input right hand worm gear drives the output worm gear in the direction such that load connected to the load drum(not shown) on the output shaft is raised, now if the motor power is shut off, the input seizes to rotate , and the load will have a tendency to move down due to gravity , thereby the output shaft tries to rotate in opposite direction, but arrangement of the worm threads is made to satisfy the self locking condition , hence the output shaft locks with the input shaft and thus the motion of the load in the down ward direction is stopped. Thus the self locking is effectively achieved