K27 rolling

[Dr. Vulcan]

I have noticed that on my K27 if it is pushed with the power off that the drivers roll. As it has a worm on the motor shaft to the gearbox this does not seem right to me. My consolidation drivers do not move when it is pushed. Has anyone else  noticed this. Is this normal for this engine or is it an indication of a problem.
The locomotive seems to run normally and has no problem pulling 15 cars on a 2 percent grade.

[glennk28]

The K-27  apparently has a steeper-pitched multiple-thread worm that allows the axles to turn the motor shaft.  Notice also that when you push the loco the motor generates power to light the headlight and class lamps. 
gj

[Barry BBT]

It is an aggressive angle on the worm/worm gear, but most importantly the worm is a double thread.  This double thread allows the motor worm and the load pushing on the drivers to interact preventing surging on down hill runs.

I had this problem about ten years ago, before finding the solution.  Typically, the rule is you cannot turn a motor with a worm drive, very true with single thread worms.

Barry - BBT

[tac]

Truly, the worm has turned.   

tac
www.ovgrs.org

[glennk28]

Old-time O Scale manufacturer Lobaugh used to make gearboxes with 3- and 4-thread worms. 

[Dr. Vulcan]

Thank-you for all the good information. Now I can enjoy my trains without worrying. I love my K27.