How to Specify Replacement Spur Gears for Worn-Out Parts?

[Ouabache]

This is outside of my area of expertise, which is why I making this post.

I'd like to replace a couple of spur gears that have worn out, but am not familiar with descriptive terminology. How may I determine what gears to order? I want to make sure I order equivalent parts. The originals are plastic, I am inclined to replace them with metal (for longevity). Is that wise? Or is choice of material, an important design consideration. (example: Is there sufficient power to turn new metal gears versus plastic?)

Here are a few terms I ran across in the parts catalogs: diametric pitch, pressure angle, face width, bore diam., pitch diameter, hub projection, hub diameter, outside diam, face width. I assume I need to measure these on my failed parts. How do you recommend I go about this? (i have already made a web search for gear parts nomenclature)

 

[Lsos]

I doubt you're going to find an exact replacement unless you get the part number and go straight to the source. I'm not sure, but I doubt gears are close to being as standardized as other machine parts (such as screws). There's just too many design variables to consider.

I worked on gearboxes in my last job. Granted, they were aerospace gearboxes, but pretty much every single thing with gear teeth had it's own separate, detailed drawings. The actual gear data was given in a chart, but it's whatever the gears were attached to that required its own drawings. Nevertheless, the gear data was quite complicated. It included the terms you mentioned, plus many others. Root diameter, TIF (true involute form), number of teeth (obviously), tooth thickness/ space width, over/under pin measurements, and who knows what else. Then there'd be other geometric features such as chamfers or radii at the tips and roots, and all these terms had their tolerances which, if they were overstepped, could very well result in adverse mating conditions.

Again, I don't know if the part you're looking for is standardized, but if it is then you'll probably just need to figure out who/ how made it, and see if they make the same thing out of metal.

 

[Ouabache]

Perhaps the tolerances on the device I am working on is forgiving. This gear is part of the mechanism inside an office paper shredder. The original failed part is plastic, (broken teeth). For a practical solution, what are the minimum criteria needed to order replacement parts? (for example: gear diameter, number of teeth, shape of teeth) ?

 

[spanky489]

Perhaps the tolerances on the device I am working on is forgiving. This gear is part of the mechanism inside an office paper shredder. The original failed part is plastic, (broken teeth). For a practical solution, what are the minimum criteria needed to order replacement parts? (for example: gear diameter, number of teeth, shape of teeth) ?

i can say from the back of my head that you will need at least 3 things: module, # of teeth and a diameter, which the name i do not know in english but looking at a picture on wikipedia its either a pitch or base diameter, one of those 2.

 

[pantaz]

QTC has a very good online tech library that may help you figure out what you need.
WM Berg is another good source.
http://www.qtcgears.com/
http://www.wmberg.com/

 

[Ouabache]

Thanks for your suggestions! With you're help, I'm getting better at
describing these gears.

 

[gmax137]

Boston Gear has a gear catalog in their "literature" that has a section on specifying gears that explains a lot of this. The pressure angle is very important (must match in mating gears). Fortunately that appears standardized as 20, 25, or 14.5 degrees in older machines. I don't know about metric gears, they could be completely different standards?

See also Shigley Mechanical Engineering Design

 

[Travis_King]

Is the shredder manufacturer no longer in business?

 

[gmax137]

Sorry for the necropost. They might be out of business by now.

 

[abert]

spur gear application and documentation

If you need a free application to create spur gears and you need some documentation regarding nomenclature, visit www.me-bac.com.

B.R.

 

[Dnomyar]

Ya got to check this out for spur gears

 

[OldEngr63]

Boston Gear has a gear catalog in their "literature" that has a section on specifying gears that explains a lot of this. The pressure angle is very important (must match in mating gears). Fortunately that appears standardized as 20, 25, or 14.5 degrees in older machines. I don't know about metric gears, they could be completely different standards?

See also Shigley Mechanical Engineering Design

Don't make this any more difficult than it has to be.

The tooth form will always be an involute; don't even think about using any other form.

14.5 degree pressure angles are long since obsolete. Modern gears are always 20 or 25 deg pressure angle.

As to what you need (assuming that this is a mating pair that is worn out) ...
1. The center to center distances must match the originals (this is the sum of the actual pitch radii)
2. The ratio of the pitch radii must agree with the ratio of the tooth numbers (to get the correct ratio)
3. Bore diameters must suit the shaft diameters
4. Face width must be adequate for the loads (if going from plastic to steel this should be easy; the other way could be difficult)
These are the essentials.

If possible, I would measure the worn out gears to try to determine the original spec. The easy measurement is over the addendum, and assuming they are standard gears, you can work back to the pitch diameter from there. The teeth are easy to count, so that is not a problem either.

After you select a pair of gears that appear to work, use their specs to check strength, contact ratio (should be > 2, if possible) and if everything is OK, order them.