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Does Your Gear Hobber Need a Friction Brake?
Cutting forces generated by a hob cutter on a helical gear are two-fold: Axial forces and tangential forces. Tangential force on a spur gear is relatively small, but when a high-helix, large diameter gear is cut on a hob that is close to its capacity, torque caused by large tangential force might be transmitted back through the gear-train of the gear hobber. It follows then, the bigger the gear, the longer the lever-arm and therefore, the greater the torque that must be restrained by index worm and gear. If torque is too large to be restrained, then index and differential gearing can start to rattle. Once the level of vibration exceeds the machines capability, anything from chatter marks and poor finish to disastrous tool breakage can occur (see pictures below).
‘Vibrate’ is not exactly the correct word to describe what happens but if you encounter the phenomenon, your initial impression will be that the gear is vibrating; initially, you may think the hold-down clamps have come loose. Actually, the gear is rotating (bouncing) back-and-forth about the central axis of the work table, where the gear is mounted. If the reader has ever been around a vibrating screen or vibrating feeder, you will recognize the ‘vibrations’ as very similar. You can easily detect the first signs of this phenomenon by looking at a thin layer of coolant on top of the gear. The surface of the liquid should be like glass. If you see ripples and in extreme cases - dancing droplets, the gear is dangerously vibrating. Of course, not all vibrations arise from the back-and-forth chatter described in this article. (Slender work arbors or pinion shafts that are long compared to diameter can vibrate also. Any vibration will compromise surface finish. - Ed)
Depending on a host of factors, this ‘vibration’ can build up, often in seconds and without warning, to a violent chatter. In extreme cases, the hob and arbor can be damaged or destroyed, see pictures below.
Hobbers with a low-ratio index worm-to-wheel drive are the most susceptible. Think of a worm gear reduction unit. If the ratio is high (say 30:1), it is considered self-locking - the load cannot drive (turn) the input shaft. But, for a low ratio, say 10:1, the load can easily turn the input shaft. Backlash between the worm and the wheel can contribute to this phenomenon, as can the helix angle and RPM of the hob. (Once any vibration sets up between the index worm and gear, self-locking effect is greatly reduced. Stability of index drive can quickly become nearly frictionless, accelerating to a possibly destructive vibration condition. - Ed)
When I first encountered this situation, my initial assumption was that there was backlash between the worm and the wheel. But even with a pre-load between the worm and the wheel, the ‘vibration’ still occurred - often without warning. One minute the hob was cutting just fine and the next instant, the gear would breakout into this chatter. After exhausting all possibilities inside the hobber, I concluded the work table had to be ‘restrained’ and the idea of a friction brake came about.
This so called ‘friction-brake’ proved to be a total solution. Not only does it totally eliminate any chatter but it noticeably improves the surface finish of any gear. This is easily seen even in smaller diameter gears:
The amount of ‘friction’ required is surprisingly small. The coolant provides all the lubrication needed. In the accompanying pictures, the draw-bolt is torque to about 15-ft-lbs. You can also put a clamp meter on the supply line and increase the no-load current by about 3% to 5%.
Fig. S1-017: High-helix without friction brake
Fig. S1-018: Same gear cut with friction brake
The Seiwa hobber in the pictures may not look like it but the machine is in like new condition. Every bit of the original hand scraping is still intact. The point being, this phenomenon can even occur on a new machine. Any implementation of this concept will have to be custom-adapted. What works/fits on the Seiwa is not likely to work/fit another hobber but the accompanying pictures provide a good starting point for anyone wishing to try this idea. (Using a friction brake as described here adds load to the driving side of the index table worm and worm gear. Normally this load is light, so it would be good to minimize friction torque to an amount that just eliminates chatter. Also, monitor heat build-up in the worm case and make sure the worm case is properly lubricated. - Ed) Information and photos contained in this article have been generously provided by: August Lehman, Managing Director Lehman Associates Manilla, Phillipines Website and contact information
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