gear pressure angle 1

[multiproducts]

We are suspect of the pressure angle we get from our gear suppliers. Documentation has been lost, etc. What is the best way to measure the pressure angle? We have an optical comparator but not a CMM. I have tried to measure the pressure angle on our comaparator but get values like 16 deg. when it should be 20 or 14.5 deg.

Thanks

 

[israelkk]

May be your gear is corrected (longer or shorter addendum)? You should first check that the outside diameter is what it should be for the number of teeth and gear diametral pitch or module. If the outside diameter doesn't fit then it may be a corrected gear. If the gear was manufactured by a hob or fellow shaper then the pressure angle can be determined from the hob data.

 

[multiproducts]

Thanks for the reply. Unfortunatly it is not convienient to visit and check on our gear supplier. We would like a method for checking pressure angle as an in-coming inspection.

 

[israelkk]

Why do you concern with the pressure angle? You didn't explain why you suspect the gear.

 

[multiproducts]

We actually suspect our supplier is using whatever hob they have sharp at the time, be it 14.5 or 20 deg. We could then run a 14.5 deg gear against a 20 deg pinion.

 

[gearcutter]

Post the gear data and I'll let you know how to measure the pressure angle accurately with a set of verniers.

 

[multiproducts]

60T/ 48 Pitch
1.277 OD
1.235 PD
1.287 OVER .036" PINS
1.185 ROOT DIA.

 

[israelkk]

I do not believe that your supplier will use 14.5 instead of 20 deg unless he is a criminal. However, how you order gears without drawings and documentations? If you suspect your supplier is unreliable then look for another one but you have know what you want and how to put it into a drawing or a document.

 

[gearcutter]

The first place to start is by measuring the base pitch of the sample. This is done with a good set of verniers and measured tangentially in the normal plane over a certain number of teeth. The number of teeth over which to measure is worked out by laying the jaws of the verniers over the "best fitting" number of teeth". By this I mean the number of teeth that allow the jaws to contact the tooth profile somewhere around the pitch line (about half way down the profile from the tip of the tooth) and not allow the tips of the jaws to make contact in the root area nor have them make contact right up on the tips of the teeth. Then take the next best fitting measurement either side of that; so in your case the first measurement is made across 7 teeth then take a measurement over 6 or 8 teeth, which ever fits best. Subtract the smaller measurement from the larger and what you have is the base pitch.
Let me know what you measure and I’ll let you know what to do next.
Is the posted data what you measured or is it the spec? Do you know the center distance and mating gear data?

 

[dimjim]

It may make sense to have a master
rack made and check the gear with
the rack. The data that you provided
indicates that the gear is cut on
a .015 smaller blank. Is the pinion
a long addendum pinion?
The pressure angle at the smaller pitch
diameter is not 20 degrees. The 20 degree
pressure angle is only at the nominal
pitch diameter of 1.250 diameter.
The pressure angle at the 1.235 diameter
is 17.9908 degrees if standard cutters
were used to cut the gear.

 

[dimjim]

You cannot measure the pressure angle
with an optical comparator unless you
know the base diameter of the gear.
You can draw the involute for the
20 degree and the 14.5 degree pressure
angle and check it at the 1.250 diameter
and the respective base diameters.

 

[multiproducts]

Thank you all very much. This is a great help. As you can surmise, I am new at the intricacies of gear cutting.

 

[multiproducts]

Thank you gearcutter. Rather than using verniers, I used our compaator to measure across 7 teeth and then six. The difference is .061". The posted data is measured. The center distance is .758". The pinion is 12/48.

 

[israelkk]

To my calculations it seems to be a corrected (loner addendum) pinion to avoid undercut and shorter addendum to keep standard center distance. There is a slight difference in my calculations but it may be explained by the tooth thinning for minimum backlash and the accuracy level of the gear. To what spec the gear are made? AGMA, ISO, DIN, JIS etc. What is the accuracy rate of the gear AGMA 10C?, AGMA 8 etc.?

 

[diskullman]

These are the 2 most standard pressure angles, and you should be able to check them against the most standard of gear gauges, available from Martin Gear in a cheap, plastic give-away item. Or you could call Globe gear & get a good set of gauges 800-523-2576

Russell Giuliano

 

[gearcutter]

By using the base pitch equation, BP = pi/DP x (cos PA), I get, for 48DP 20deg PA, 0.0615". For 14.5deg PA the BP works out to 0.0634". Based on your measured results your gear is 20deg PA.

 

[multiproducts]

Thanks, gearcutter, I have found it difficult to get repeatability. Being off just a few thousands on the measured distance cna throw you off 5 deg.

We cut our gears to AGMA6 and deviate from the formula for backlash. Will the pressure angle be different if the OD and PD are under formula? We do have a set of Globe rack gages.

 

[israelkk]

This seams very odd to cut 48 diametral pitch gears to AGMA 6. It is very common to cut them to AGMA 10 without real price increase. Every PIC, SDP or Bergs standard off the shelf gears are cut to AGMA 10. We even custom designed and manufactured AGMA 10C gears cheaper than off the shelf gears depends the quantities are in hundreds and thousands.

The pressure angle has nothing to do with the OD and PD.

 

[gearcutter]

As long as the generating tool's pressure angle doesn't change then no matter what changes are made to the blank, the generated PA will remain the same.
But, touching on some of dimjim's points, if you apply a correction to the gear, (either a plus or minus correction) even though the generated PA remains the same, the operating PA can change. This is why gears with a large plus correction can look as if they have larger generated PAs than gears without the correction. The operating PA could be measured but you would have to include information about the center distance and mating gear; generally though, it is calculated.
Now it gets really confusing.....even though the operating PA may change, the base pitch will remain exactly the same no matter how much correction is applied to the gear proving that the generated PA is a constant which relates directly back to the tool used.
Why do you have the concerns you’ve mentioned? Are you having problems with the parts being supplied?

 

[multiproducts]

We are having problems with the parts being and we have bins of unidentified gears that we are trying to identify, specifically the PA. When measuring across the teeth and using the difference to calculate PA how do you determine the number of teeth to measure across? If I measure across 7 and 6 teeth for a 58/48 gear and then across 5 and 4 teeth I will get a different PA. Thanks