helix angle different between wheel and worm gear

[engrenage]

Hello everybody
I have a question about worm-wheel gear pair.
In Germany very often there are difference on the helix angle and the pitch between the wheel data and the worm data.
For example I am working on a small gear box for automotive and I have:
- Helix angle for the wheel: 12.938°
- Helix angle for the worm: 13.50°
The normal module is the same (0.7244)
The theoretical axial pitch is 0.7244 x PI =2.2757
The pitch on the worm drawing is 2.2810

It is something I found in lot of gear definitions.
Does anybody know why there are these differences?
Thanks in advance for your replies.

 

[jeffcannon]

engrenage,

I too have been working on a German right angle gearbox and had similar questions. I was never able to get satisfactory answers, but in our application the main purpose seemed to be to provide the ability for the gearbox to be backdrivable. The angles and contact ration are very critical if you are trying to backdrive a worm. Is this the case with your unit?

 

[spigor]

Could you please post the remaining data for the members (e.g. no. of teeth for the gear, no. of starts for the worm, diameters etc.)?

 

[gearguru]

At what diameters are the helix angles measured?

 

[Spurs]

I have heard of this practice many times although I have never been able to get first hand confirmation of it either.

The advantage that I have heard (2nd hand) is that it is supposed to be to able to prevent lockup of the drive under shock loading.

That being said, it really doesnt tell you why the lockup was happening in the first place ie - bending of worm, misalignment of axis, slop in bearing system, etc.

Some people believe that by offsetting the angles, more backlash is created. This is an incorrect assumption since any deviation in angle will consume backlash. If backlash is all one needs, simply thin the teeth.

If anyone can shed some real light on this issue, it would serve to educate all of us.

 

[mfgenggear]

The only factor I can think of is to allow for deflection caused by force. That is a total assumption.
I have seen it drawn on customer prints. but it is not common.

I cannot believe it is a good to purposely change the helix angle.

MfgEnggear

 

[spigor]

Another possibility would be controlling of a lubricant wedge, but since no more data has been posted I'm unable to verify.

 

[engrenage]

Sorry for my late answer and many thanks for your previous replies.
The applications where I found this problem very often are for european automotive market.
It is the case for wiper motor, window lift motor and seat motor.
The wheels were made of plastic and the worms in steel.

For example, the data of a wiper system:
-nb of teeth wheel: 69
-nb of teeth worm: 1
-normal modul: 0.916
-normal pressure angle: 11°
-wheel tip diameter: 65.4 mm
-worm tip diameter: 8 mm

-wheel helix angle: 8.648°
-worm helix angle: 9.633°

 

[plasgears]

If you lay the worm on the gear, any mismatch of angle will be evident. There is no alibi; just design or mfr error. I had a production design designed by others that was terribly noisy. Mismatch of angle was about 5-10 deg. The noise went away after redesign.

 

[spigor]

I could not check in full, too little data, would need the actual drawings. However, it's already safe to say that it looks like the mesh should be OK. The parameters are not as specified by the standard DIN 3975, either to confuse, or for some other reason. That's all there is to it.

 

[mraabe]

Sometimes the calculation of worm gears is done as crossed helical gears, where worm gears are a special case.
For crossed helical gears a profile shift can be applied for both gears, which can be useful to have a thinner metal worm and a wider plastic worm gear.

If the sum of profile shift factors is not equal to zero you will get different helix angles on generating and operating pitch diameters. The sum of helix angles on operation pitch diameter has to be equal to the axis angle (often 90°).
Therefore the sum of helix angle of generating pitch diameters is not equal to the axis angle if sum of profile shift is not equal to zero or the lead angle of the worm is not equal to the helix angle of the gear.

This difference has to be considered if the gear is not manufactured with a worm like tool but as a helical gear.

So if the design is done as crossed helical gears with sum of profile shift not equal zero you will see these different angles.