Symbolically, gears are the most mechanical of all things engineering. The germans more accurately call them zahnraden or toothed wheels, which is exactly what they are.
Gears are divided into two groups: one transmitting power, one transmitting motion! Well, obviously one does the other too, but the former are carefully designed and put into well-sealed gearboxes. Failures of drive packages on the gearbox side are more often than not results of contamination, leakages, wrong lubricants, overloads, vibrating loads, electric current passing through the gearbox, mounting screw-ups like misalignment, etc. Too much radial load (often by an over- tightened/over-designed V-belt), or too great a radial load overhang - pulley not being as near to the output side bearing as it could be/ should he (squeeze it inward to the very last possible millimeter!) as possible, commonly results in shaft breakages. This breakage usually takes place just inside the shaft-side bearing, not outside it. This is inevitable. If the gearbox is driven by a V-belt, this usually happens to the motor.
Worm shaft / worm wheel teeth get very badly chewed up by overloads, wrong mounting, leaking oils, wrong type lubricants, etc. as the manufacturers will tell you. One factor no manufacturer will talk about is the presence of loose sand (if it is a cast-iron box) and/or metal chips in brand-new factory-fresh gearboxes. This stuff gets between the teeth, and wears them out. The debris thus produced only accelerates the process.
Lets clear some of the common confusion about them.
1. Involute gear action is not pure rolling. Before and after the pitch line, there is a very small amount of sliding.
2, As number of teeth decrease in a spur gear, the tooth root goes on thinning, and soon the tooth takes the shape of a classic Coca-cola bottle, rather than the healthy silhouette of a Sumo wrestler. To counter this, you should apply correction. This essentially makes the teeth stronger. Correction is simply cutting same number of teeth of the tooth depth on a larger or smaller than normal blank with the very same hobbing machine settings. For example, the normal pinion of a 2 module, 15-tooth gear with has a pitch diameter of 30mm, and an OD or addendum dia of 34mm. If you increase this by one module per side, you end up with a gear of 34mm OD, and this is called a correction factor of +1 (module). If this OD is 37mm, this factor will be +0.75. 36mm will be +0.5. 32 mm will be -0.5 and so on. Pinions smaller than 17 teeth need a positive correction. Now if this center distance is standard, pinion has a +0.75 correction and the mating gear is 60 tooth, then it must have a -0.75 correction, and its OD will then be 121mm. Negative correction in a gear with many teeth does not alter the profile detrimentally. With +1 (less with more teeth) correction, you can safely go down to10 teeth. Below that, down to 3 teeth, is the domain of super- specialisation.
3. There is no need to over-cut gears to get sufficient backlash, and vise a versa. No need to increase CD to do the same too.
4. And proper depth (exactly two times module) must engage in all situations - absolutely no exceptions there.
5. You can mate positively corrected (enlarged) pinions with uncorrected gears if you adjust the centre distance accordingly. You can mate to positively corrected pinions to with increased CD.
6. Helical gears are not necessarily less noisy. Properly made and mounted spur gears can be quite silent too.
7. Avoid designing mouths with adjustable CD, like putting the two shafts in sideways - adjustable pillow (plumber) blocks.
8. Stay away from cutting gears on milling machines with form cutters and indexing heads.
9. It is not at all a healthy idea to deep-mesh gears to avoid backlash. If you need backlash-free operation (say for a servo-drive), use timing belts instead.
10. Herringbone gears are only used in gigantic gearboxes. Otherwise, they are just a red herring.
11. You can mate helical gears with worms for light loads, the helix angle being the helical angle of the worm thread, both having same hand, just as in worm wheels. Nylon helical gears mated with steel worms work very well - open a Lucas TVS wiper motor and see for yourself. Toys often have sheet- metal spur gears happily mating with worms.
12. You can even create a cross-helical right-angle drive with two helical gears of the same hand and it is not so sensitive to CD errors.
13. Avoid straight bevels. They are a nuisance to make one-off, prone to design and manufacturing mistakes, sensitive to mounting rigidity and demand great accuracy, and are noisy even at 100 rpm. Pick up new standard hypoids or spiral bevels from the auto spare market and use them.
14. Planetary gears require very accurate gears and even more accurate planet carriers. They must be enclosed properly. If 3 planets are used, all numbers-of-teeth must be divisible by 3; if -4, then by 4 - you cant break this rule. Internal gear ring, if not very large, can he made by wire cut. Be very clear in your mind about who is driving whom, their directions of motion, torque reactions, bearing loads and reduction ratios.
15. Spur gears, which are not deeply chamfered on a proper tooth-chamfering machine, will NOT slide into each other axially either while moving or even when stationary. Chamfering on lathe does not solve the problem. If sliding is needed, and if you cant get tooth- chamfered gears, use a constant-mesh design with trapezoidal dog-clutches.
16. Harder gears can tolerate more contact stresses; so overall sizes can be reduced if you harden. Pinion must be harder than the gear. If the stresses are high, do a dye-penetration test for cracks.
17. Very high reduction ratios can be obtained by many ingenious designs.
18. Before you issue production drawings and write the process sheet, talk to your hobber whether he will do that job with a topping hob or a non- topping hob. As the name suggests, topping hob shaves the OD too. Non- topping hob leaves it alone. So your blank should have machining allowance if a topping hob is going to be used, and should be made to final size if the hob is non-topping.
19. Gear arrangements are one area where one can inadvertently get into an impossible-to-assemble design. So, after you detail, work out the assembly sequence or call the assembly guy over to get his approval.
20. Not all gears carry heavy loads, nor all run continuously. Some only work a bit for a few seconds. So keep the materials, processes and design sensible and dont go overboard, nor cut too many comers.
21. If gears are failing prematurely, have a good look at the failed gears and try to figure out precisely why they have failed.
22. If all teeth are showing signs of distress, like pitting, scuffing or heavy wear, chances are the bearings are finished too.
23. If a gear, pulley, etc. is stuck on a shaft, and you cant put a proper puller to it, but can only pry it out with a lever bar, then pry it out with two identical lever bars (or two identical screwdrivers) put opposite (mirroring) each other and with a jerk, force it out as symmetrically as possible, and see the magic.
|Posted : 8/13/2005|