Making spare gears from existing: how to spec
Making spare gears from existing: how to spec
(OP)
We have some old equipment from the 1940s, slow speed, intermittent service, outdoors.
We are undertaking to make drawings of all parts so that we can have spares fabricated. For the gears, our drawings show the overall (outside) diameter of the gear, the rim, spoke, and hub dimensions, and the number of teeth. The gears are castings.
We are sending out a sample gear to find out the tooth profile (involute vs. cycloid) and the pressure angle.
What other information would we need to get off the old gears to fully specify the new spares? What info is needed to give to a gear manufacturer?
We are undertaking to make drawings of all parts so that we can have spares fabricated. For the gears, our drawings show the overall (outside) diameter of the gear, the rim, spoke, and hub dimensions, and the number of teeth. The gears are castings.
We are sending out a sample gear to find out the tooth profile (involute vs. cycloid) and the pressure angle.
What other information would we need to get off the old gears to fully specify the new spares? What info is needed to give to a gear manufacturer?





RE: Making spare gears from existing: how to spec
I am assuming these are spur gears which are fairly straightforward to replicate. I suggest getting a copy of "Machinery Handbook" and referring to the section on gears. Err on the side of caution and list everything you mentioned along with pitch diameter and circular pitch. Also, you probably can get them machined cheaper than having them cast unless the casting provides you with important characteristics. Machine shops circa 2000 have a few advantages over their 1940's counterparts.
RE: Making spare gears from existing: how to spec
for 1940's gears, they're normally involute.
pressure angle is usually 14 or 28 degrees... if I remember right :)
hope i've been some help
RE: Making spare gears from existing: how to spec
The American Gear Manufacturer's Association has a good website - check them out to obtain standards and information.
RE: Making spare gears from existing: how to spec
http://www.falkcorp.com/
RE: Making spare gears from existing: how to spec
To get this data you need to measure the dimension on K teeth (number K depends on DP & number of teeth)
If you measure the dimension on K teeth and on K+1 teeth you can calculate, for a given pressure angle, what is the diametral pitch :
DP = PI* 25.4*cos(pressure angle)/(Wk+1 - Wk)
If the pressure angle is not exact it does not matter : you only need the base pitch. You can cut a gear with a different pressure angle than the original as long as the base pitch of the cutter is equal to the one of the part.
If the teeth are too small or the helix angle too high to measure the K teeth dimension, you need to measure the dimension over pins... and then use a gear calculation software to give the manufacturer the addendum modification coef. you need.
RE: Making spare gears from existing: how to spec
Don't forget material type, machined-finish [microinches], surface hardness and any platings or treatments [IE: nitride, carburization, flame hardening, etc].
Is balance an issue [for higher speeds]?
Regards, Wil Taylor
RE: Making spare gears from existing: how to spec
1. Measure center distance between the gears; lets call it "a"
2. Calculate "m" like this: m=0.5*a/(z1+z2)
If you measured "a" in mm, than "m" is the module of gears and using it you can calculate the rest of gear data (addendum = m; dedendum = 1.125*m, just to show some basic calculations in mm)
For those who prefere dia pitch to module:
module=25.4/dia pitch
(module is in mm, dia pitch is in 1/inch)
and you can go ahead with the inch based gear calculations.
gearguru
RE: Making spare gears from existing: how to spec
FrenchCAD
Université Joseph FOURIER
Département Génie Mécanique
Grenoble
France
cyril.guichard@wanadoo.fr
RE: Making spare gears from existing: how to spec