Making spare gears from existing: how to spec 5

[KM]

We have some old equipment from the 1940s, slow speed, intermittent service, outdoors.<br>
<br>
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.<br>
<br>
We are sending out a sample gear to find out the tooth profile (involute vs. cycloid) and the pressure angle.<br>
<br>
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?

 

[Nate]

KM:<br>
I am assuming these are spur gears which are fairly straightforward to replicate. I suggest getting a copy of &quot;Machinery Handbook&quot; 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.

 

[rgv]

You would also need the root diameter. You would then know the tooth height after subtracting the diameters and dividing by 2. You can establish the tooth profile whether it is of the snub tooth type or the normal type. The tooth height is the total of your addendum and dedendum. From here, you will be able to get your pitch diameter, then your diametral pitch. Since the values you'll be getting are not exact, the values should be reverted back to their nearest standard values since these are sure to be standard when the gears were first designed.<br>
<br>
for 1940's gears, they're normally involute.<br>
pressure angle is usually 14 or 28 degrees... if I remember right <br>
<br>
hope i've been some help

 

[MikeVV]

Machinery's Handbook provides the round-pin inspection data that will be useful for verifying the gear tooth characteristics.&nbsp;&nbsp;A &quot;standard&quot; round pin is inserted between two teeth.&nbsp;&nbsp;An identical pin is inserted on the opposite side and the outside diameter is measured from these pins.&nbsp;&nbsp;This is used to check the gear (dimensionally) and &quot;standard&quot; gears are listed in the reference.&nbsp;&nbsp;Compare values to see if your gears meet the standards.<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The American Gear Manufacturer's Association has a good website - check them out to obtain standards and information.

 

[BigAL]

You might want to contact the Bridge tender group in New Bedford, MA. They were replacing the gears on the swinging bridge there. I believe they were using FALK out of Milwaukee.<br><br><A HREF="

http://www.falkcorp.com/"

TARGET="_new">

http://www.falkcorp.com/</A>

 

[julio]

You also need to give the tooth thickness :this will allow the gear manufacturer to determine the addendum modification coefficient in order to cut a new one.
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.

 

[WKTaylor]

KM...

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

 

[gearguru]

Why to replace only one gear? There are always at least two of them working together and they deserve to be replaced at the same time, especially if they served that long! In this case I would design both new gears based on known dimensions of the existing ones: You obviously know the numbers of teeth on your gears. Let's assume they have &quot;z1&quot; and &quot; z2&quot; teeth:
1. Measure center distance between the gears; lets call it &quot;a&quot;
2. Calculate &quot;m&quot; like this: m=0.5*a/(z1+z2)
If you measured &quot;a&quot; in mm, than &quot;m&quot; 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

 

[FrenchCAD]

Agreed. And way simpler. FrenchCAD
Université Joseph FOURIER
Département Génie Mécanique
Grenoble
France
cyril.guichard@wanadoo.fr

 

[gbent]

What are the usages of these gears? Approximate loading and speed ratings, consequence of failure and cost and value are also important. Since these are cast iron gears stored outside, I assume they are not precision parts. Don't waste money on 4 digit tolerances if it lives outside. Almost guaranteed, origional prints were dimensioned in fractional inches. Use old handbooks when making specifications. Steel replacements will be much cheaper than cast iron, but will not weather nearly as well outside. However, this depends greatly on where you are located.