Guys-
First off, thanks to all of you that contributed suggestions and hints in the rebuild of my QA42-A snowthrower gearbox repair. This helped very much.
Gearbox is now all back together and the snowthrower seems to work better than ever. The inordinate amount of time spent in rebuilding this was due to the difficulty in disassembly – a rather ham-fisted mechanic was in this thing previously and made the job much more time consuming than it should have been. As stated earlier, the drive key on the pinion bevel gear had sheared, rendering the gearbox in a permanent “neutral” condition. Disassembly showed that ample grease was present with no missing gear teeth – a good sign. Grease was removed with a wooden spoon using a cap from a spray paint can as a receptacle. Note that the amount just about fills the cap.
After removing the snap rings from both ends of the input shaft, it was not possible to drive the shaft outward (using a brass hammer) as the pinion gear would not slide over the end of the shaft. Driving the shaft inward would not work either as binding of the shaft on the ID of the bearings resulted. It was obvious that this had been apart before and someone had beaten on the end of the shaft with something rather hard and mushroomed the end, thereby preventing the shaft from sliding outward through the pinion. About 30 minutes of careful work with a hand grinder removed the obstruction and finally allowed the shaft to be driven outward.
Once apart and after cleanup in the wash tank, the shaft and pinion appeared as shown:
A new key was made up (.188 X .188) from stock on hand. Both the gear and shaft keyway were in good condition. However, there was scoring on the shaft (and some traces in the ID of the gear) in an area where neither the gear nor the support bearing are located.
After a little detective work, I concluded that the previous failure was caused by the shaft moving inward (due to the outer snap ring not being in its groove) thereby allowing the gear to be disengaged from the key, slipping on the shaft, and galling both gear and shaft. The previous ham-fist’s fix was to pound the shaft outward with a steel hammer, thereby engaging what remained of the key with the gear and refitting the outer snap ring (which I found was bent and not suitable for reuse).
Some careful filing and emery cloth polishing was performed to clean up the scoring, but the pinion gear would not slide onto the shaft with what should be a line-to-line fit. I then clamped the pinion in the vice between hardwood blocks and applied fine-grade valve lapping compound on the shaft and ID of the gear. Using the pulley fitted to the shaft as a hand wheel, I lapped the shaft and gear such that a very nice sliding fit resulted. The ID of the pinion cleaned up well too.
All gears in this gearbox are fully machined (not powdered metal) and the contact pattern on the teeth showed a very respectable pattern with no signs of debris passing through. The shaft material is not very hard though and easily damaged by using steel hammers or drifts. Only soft brass or plastic should be used for tapping on the shafts when removing or inserting them (an arbor press would be ideal).
Reassembly with the new key and 2 new retaining rings (which luckily, I had in stock) was easy with all the proper fits reestablished.
Rotation was smooth and the backlash (which is non-adjustable) seemed to be about right by eye-balling it. Not quite as precision as a helicopter tail rotor gearbox, it nevertheless is perfectly suitable for snowthrower duty.
Per Steve B’s recommendations, I mixed up a concoction of Valvoline Moly Disulfide grease mixed with 75W-90 gear oil in a ratio of 3:1 respectively. A new spray paint cap was used for a container to get the proper quantity to match what was removed. The resultant mix would still flow by gravity but was viscous enough that the term “grease” was still applicable. This was then loaded into the gearbox and cover was replaced. While the box was still in the un-cleaned state, I chased all the threaded holes with a 1/4X20 tap. The cover is now held down by stainless cap screws and star washers, disposing of the cheesy self-tapping screws that were original equipment.
She’s all back together and working perfectly. Again, I’m very appreciative to all that contributed their help to this rebuild. Thank you my friends!
