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Field measurement of helical helix angle of ball mill gears

tensor47 (Mechanical) (OP)
5 Jul 09 22:11
A client has a second hand Sag mill with no detail drawings of the mill pinion and girth gear.
The mill pinion has not been turned around so it has one unworn side.

To manufacture a new pinion I am advised that the pinion must be removed, taken to a the manufacturers, and accurately measured on their 3D machine, and then returned and replaced in the mill.

Is there a way to measure the helix angle & pressure angle sufficiently accurately in situ to save the 3 days required for all this work ?

Pressure angles seem to be 14.5 or 20 or 25 deg. so will a rough measurement that clearly seperates the possibilities be sufficient ?

Do helix angles come in significant steps like this which could be sorted into one value or another without needing micron accuracy, or do they come in "any" angle requiring high precision meaurement ?

I can imagine 2 measurement ways :-

a/
A flat plate with an aligning strip on one side to rub against the side of the pinion, to keep it square while rolling the plate around several teeth of the pinion.

Put 3 stips of non shrink grout on the plate (edges and centre) and this will be sqeezed into the rack shape which is now flat and can be measured on a 3D machine.

The pressure angle would be easy to measure with a protractor as we have a straight sided rack profile, if minimal accuracy is acceptable.

For helix angle only 10 mm thickness of non shrink grout is needed to define the unwoun top section of the tooth, to minimize distortion of a full depth moulding.

If the plate slips or if the helix angle only allows for a slight overlap of gears the plate may not roll smoothly.

b/
For the helix angle a thin steel ruler which twists easily to take up the shape of the cylindrical curvature and can run along the tooth face say 10 mm down from the o.d. comes to mind.

Is such a line of contact actually straight to the twisted ruler ?

If it is then :-

Imagine three bridge pieces to reach from tooth top to tooth top located by a single small screw at 0, 150, and 300 mm along the ruler, each about 10 mm deep, allowing the ruler to be twisted at these 3 places to the tooth tops (matching the circumference curvature of the gear) and also having it's edge (10 mm below the tooth tops) running along the unworn face of the tooth.

To measure the angle at the 0 end an additional straight edge also bolted loosely by the same small bolt would be pressed against the side of the gear, then the 0 mm screw tightened, and the assembly removed and allowed to become flat. The angle between the 2 edges would be the helix angle. Or would it ?

The straight edge to be pressed against the side of the gear would need an extension to reach the 0 mm screw which would be slightly inside the gear edge.

What do you think ?

Cheers

John

gearcutter (Industrial)
6 Jul 09 1:23
Unfortunately there is no way to accurately measure the helix angle without specialized equipment.
What you are trying to measure is a curved line wrapped around a cylinder so using straight edges etc. is pretty much futile.
There are ways of coming up with something close but you run the risk of not being able to correctly assemble the parts if you even slightly get the angle wrong.
The only way to accurately determine the angle is either on a hobbing machine, gear grinder or a C.M.M.
There are portable gear C.M.M.s around. I'd get in touch with a reputable gear manufacture and find out if they have a portable lead & involute checker.
If you know the normal pitch then the pressure angle is easily determined by measuring the base pitch. Verniers are usually all that is required to measure this.
You cannot determine what the pressure angle is by using a protractor.
 

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181
 

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