Cam gear broach accuracy

[Hasbro]

Some time ago I ran into information on the problem issues of correct cam gear broaching. Seems to be a major problem that is not realized in the engine field. Apparently the broach pitch & depth is all over the map, so much so that a grade had been attached to the shafts & the good are few. There is a casting company that has invested in new equipment to correct this issue although I can't recall who they are. Their cam cores are very expensive in comparison to the jeneric cores too. This also applies to the dist drive gears. ------- What say thee?

John Haskell
Aire Research Engr.

 

[TOHCan]

That's why I always degree cams.

 

[icrman]

If the pitch was incorrect something else would be wrong other than just location. Gears would bind, chains would jump.

 

[magnograil]

Are you talking gear teeth or woodruff key? Gears are for the most part cut on hobbing machines. The pitch is fixed by the gear ratio between the cutter and work. A single start hob will turn 20 times for a single turn of a 20 tooth gear blank. The variability comes in setting the depth of the cut which effects the backlash of the gearset.
The keyway is usually indexed to a single tooth. Unless the index does not hold the gear consistantly or the broacher has slop, the keyway should be consistant.
Only the width of the keyway matters, not the depth.
Offset keys are available to fine tune the gear to cam relationship.
Whether they can grind all the cam lobes to the correct index is another matter. There you have to split the difference on all the lobes.

 

[EngJW]

It could be that the broaching process caused tooth profile errors. The tooth form is an involute curve, so the types of errors that occur include deviation from the perfect form, spacing errors between teeth, lead (tooth alignment with axis), and runout. There are AGMA standards that allow different amounts of error. The lower numbers around 6 or so would cover crude or low cost gears that are just hobbed. Higher numbers like 10 are for precise gears that are hobbed, maybe heat treated, and then finish ground. Engine gears should be around 10.

These errors lead to vibration and wear.

Other processes like milling, broaching, plastic molding, or powder metal are used, and each would produce gears of a certain quality.

 

[Hasbro]

The issue was in the hobbing process.

John Haskell
Aire Research Engr.