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Thread: Need help with a gear
10-12-2009, 03:09 PM #1
Need help with a gear
I have a gear that i need to replace but i don't know much about gears.I would like to buy gear stock could you guys tell me what to measure on the gear and where to look online to buy gear stock.
10-12-2009, 03:20 PM #2
How about a relative size for starters...? clock pinion or drawbrige pinion?
10-12-2009, 03:23 PM #3
Take a look at the Boston Gear site www.bostongear.com
You can download their catalog, or look at it online.
They have a good section on gear terminology and design, near the end of the catalog.
The fundamental question about gears is the "pitch angle". Common values are 14 1/2° and 20°. Gears of different pitch angles will not work together, so you need to know what variety you're replacing. This parameter is not easily measured.
Next most important is probably "diametral pitch" (abbreviated D.P.), which is the number of teeth in one inch at the pitch circle diameter. For a 12 D.P. gear standard, a 12-tooth gear would be 1" in diameter, and a 36-tooth gear would be 3" in diameter. As with pitch angle, mating gears must have the same diametral pitch since it determines the width of the tooth and the gap between teeth.
You'll encounter the term "pinion". There's nothing special about a pinion. It's just the smaller gear of any pair.
You'll find "pinion rod" in tool catalogs. These are rods with teeth cut axially. You cut off whatever length you need, then machine off unwanted teeth to form shafts, bushings, or other features. It's easier and faster than buying a small gear and fitting it to the other parts.
10-12-2009, 03:33 PM #4
(My reply presumes that this is a regular straight spur gear -- if it's not, there's potentially a bunch of other parameters that need to match up)
Couple things have to match up:
1. toothform -- probably involute, but there are others
2. pitch -- either diametral or metric module
3. pressure angle -- usually 14.5* or 20*, but there are others
4. # of teeth (duh)
After that a couple things have to be "at least" or "no more than":
1. tooth width has to be at least what the original gear was, you can cut off the excess
2. bore has to be no more than what the original gear was, although you could sleeve and rebore if it's too big
Finally, you have to cut a keyslot if the original gear had one. Most off-the-shelf gears are supplied without keyslots, and with a hub that needs to be cut off. If this is an idler, you might have to install a bronze sleeve.
You ought to match up the material, unless you have a good reason to change. A lot of off-the-shelf gears are cut from 1144SP steel, but you can also get alloy steel, hardened and ground, bronze, and others.
You can buy stock spur gears from McMaster-Carr and MSC.
10-12-2009, 04:33 PM #5
You can nail down tooth size (diametral pitch if imperial parts) without much fooling around.
1. Count teeth
2. Add two to above
3. Measure OD of gear with a mic
4. Divide sum of #1 and #2 by measurement #3
34 divided by 2.125 = 16 DP
10-12-2009, 06:00 PM #6
It has 15 straight teeth with a diameter of 1.054"
10-12-2009, 06:02 PM #7
Heres another view.
10-12-2009, 06:21 PM #8
OD a little off due to odd number of teeth, but its a 16 DP 14 1/2 degree pressure angle.
10-12-2009, 06:35 PM #9
Boston Gear makes a 14 1/2° 15-tooth 16 DP, 1/2" face width, catalog number NB15B, on catalog page 9.
They also make a 20° gear, other specs the same, catalog number YB15, on page 31.
Both of these are steel. Nothing in brass.
Check the catalog pages for full dimensions including hub and bore.
Applied Industrial Technology at www.applied.com is a Boston Gear distributor. They have the NB15B listed as "available" under their catalog number 893732 at $15.34. "Available" probably means drop-ship from BG.
The YB15 (#894352) is listed as "in stock" at $16.63.
You can probably find these at other distributors also. I checked McMaster-Car and they don't have any gears with these specs.
10-12-2009, 09:27 PM #10
This is a sterling example of a picture is worth a thousand words. The pressure angle is easy to discern to a practiced eye. (yes, there are other pressure angles than 14.5 and 20 degrees, how often are they encountered outside of max effort such as aerospace?)
Many questions are asked here without an easily supplied photo which would make the question so much easier to answer.
10-12-2009, 09:47 PM #11
10-12-2009, 10:41 PM #12
10-13-2009, 01:34 PM #13
10-14-2009, 03:18 PM #14
This place is great thanks for helping me out with this.
10-14-2009, 04:11 PM #15
10-14-2009, 04:41 PM #16
Not so Liegh, I read the entire thread up to the post before mine. I saw the photos and Johnoder's response. My point was after the first post with no photos the general responses were trying to be helpful but didn't have enough information to nail down an answer. After the two photos were posted with an OD, tooth count and description of straight teeth an immediate and accurate answer was supplied by Johnoder. My hat is off to him.
My post was meant as a jentle prod for OPs asking questions to provide as much information with the question as possible and if applicable, photo(s) to facilitate getting good answers.
The dynamics of this thread were a very good example of it.
10-14-2009, 04:45 PM #17