Why does a gear have extra holes?

Question

In a gear you normally have a center hole for the shaft to be put into. Why do some gears have holes surrounding that center hole?

Is it for weight reasons? Does it add stability? Is it so you can add special parts?

I think it's safe to say that the holes do not _per se_ add stability – a solid wheel of the same thickness profile will certainly be more stable. Just, and that's probably the point, it won't be _much_ more stable, yet significantly heavier. And the holey gear will be more stable than a solid but thinner gear of the same mass. — leftaroundabout, Dec 11 '20 at 09:30

NMech · Accepted Answer · 2020-12-10T16:54:10.827

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Different gears have different drivers for having holes.

One way you can categorize gears is by whether they are used primarily for transferring:

motion: transferring angular position and angular velocity (see clockwork)

torque: when the gears are used in power transmission

Motion

Gears that transfer motion don't need to transfer power. So their strength is not usually important. The main parameter is their diameter which determines the kinematic behavior of the gear train. Because of that:

they don't need too much mass to sustain the loads.
its preferable that they are lightweight (see clockwork).
having additional mass would increase the inertia and therefore increase the forces in the system.

So in this case the primary driver is usually reduced mass of the system and reduced mass inertia. The reduced mass of the gears leads to lighter and more compact mechanisms.

Transmitting torque.

These gears are usually encountered on motor gear boxes. It is noteworthy that you may have torque transmission without any angular motion (when the torque exactly counteracts the load). That torque generates forces and stresses on the gears.

So for gears that are subjected to high forces and transmit torque (and power), removing material and opening holes doesn't make much sense, because essentially you'd be weakening the gear.

So, the main driver in this scenario is for high volume runs in which some type of casting can be used to reduce the material used. However that is only true when the process is some type of casting. If a material-removal process is used to manufacture the gear, then economy of the material is not a valid reason, because the expense of using cutting tools and recycling the swarf would be greater than the value of the material saved.

Secondary reasons for this type of gear are:

to reduce their moment of inertia.

This is especially true if they change speed quite often, because a high moment of inertia would require high torque to change the speed quickly, or longer times. High mass moment of inertia results in the gearbox being less responsive to changes.

Splash lubrication

Especially in the case of gearboxes with high losses, it is desirable to remove heat as fast as possible. A very common way is to use lubricant to enhance the heat transfer through convection. If the linear velocity of the gear teeth is high (at least 3 m/s), then it is possible to use the gear itself to create a lubricant mist inside the gear box (the alternative would require an oil pump which would spray the lubricant). See image below for an example arrangement. For the method to be successful, it is imperative that at least one gear to be partially immersed in the lubricant fluid).

The extra holes in the gear would help by creating more mist in the gearbox, and also by providing greater surface for the convective heat transfer interaction.

reduced weight

Again reduced weight leads to lighter and more compact mechanisms.

edited Dec 10 '20 at 16:54

answered Dec 08 '20 at 16:06

NMech

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While I think I understand what you're trying to say here, I find your phrasing kind of confusing, and I can easily see how someone could wonder how a gear can "transfer motion" without "transferring power" or _vice versa_. (Of course, the answer is that all gears transfer both motion and power, but the ratio of the two — i.e. the [torque](https://en.wikipedia.org/wiki/Torque) — can vary a lot. And it's the torque that determines how strong the gear needs to be in order to withstand it.) – Ilmari Karonen Dec 09 '20 at 01:27
You are right, that the phrasing is confusing. However, I was having problems with finding the right word to replace motion. I've made an attempt, and I hope it makes more sense now. – NMech Dec 09 '20 at 06:13
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I don't know how much of a factor this is, but I'd also think that cutting holes in the gear increases the surface area, which can reduce the heat buildup vs. a solid gear. Gears are subject to a lot of heat due to movement and friction with other gears, so this could be detrimental. The holes also allow for airflow (or lubricant if they're submersed), and and the spokes can act as a fan and provide convection to cool down the gears and anything around them a bit. – Darrel Hoffman Dec 09 '20 at 15:24
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Another reason is so that you can slip some sort of lever into the hole to hold the gear steady or force it to move when working on the mechanism. – Hot Licks Dec 09 '20 at 15:26
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They are also used to transform torque or angular motion into the reciprocal versions linear momentum or linear motion. That would require "holes" off-center. – Stian Dec 10 '20 at 11:59
Any gear that is allowed to rotate, even by the tiniest amount, is a gear that transfers motion. And, a gear has no reason to exist if not to transfer torque. I think what you are getting at is that some gears are expected to transfer _much less_ torque than some other gears of similar size. – Solomon Slow Dec 10 '20 at 18:34
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yes the key word is **primarily** . For example, there is not much torque transferred in clockwork gear. What matters is the precision of movement and angular velocity. I couldn't find a word that describes this accurately. Any help is appreciated. – NMech Dec 10 '20 at 18:37
Could holes also be used to add flexibility, and thus some shock-absorbing capability, to the gear? – Andrew Morton Dec 11 '20 at 16:28
actually, IMHO that would be a really bad idea. Any elastic deformation would result in stored energy as a spring, which when released would be like a slingshot. If you need shock absorbing for safety, the best idea is to use a either a slip clutch or a pulley with a belt somewhere along the gear drivetrain system. – NMech Dec 11 '20 at 17:23
@NMech I was thinking more along the lines of how a dual-mass flywheel works. – Andrew Morton Dec 11 '20 at 18:33
@AndrewMorton are you suggesting to essentially make the gear with the holes the one half of the dual mass flywheel? – NMech Dec 11 '20 at 18:46
@DarrelHoffman Are they really subject to "a lot" of heat? Some, yes. I'm sure giant big strong gears can be subject to a lot. But most gears? – user253751 Dec 11 '20 at 19:44
@user253751 That admittedly only applies to high-speed, high-torque gears such as those in engines. Clockwork gears don't really have this problem of course. – Darrel Hoffman Dec 11 '20 at 19:48

Solar Mike · Answer 2 · 2020-12-09T11:16:59.913

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In addition to the other answers such as weight reduction and inertia, there are other possibilities:

Often there can be a precision machined hole for timing purposes. A common solution for some / many internal combustion engines to get camshafts timed to the crankshaft. Of course, dial gauges may also be used.

Also, there can be threaded holes to help with removal especially when space is limiting the use of normal gear pullers.

edited Dec 09 '20 at 11:16

answered Dec 08 '20 at 13:54

Solar Mike

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Could you expand on the "timing purposes" sentence? – DBS Dec 09 '20 at 11:13
@DBS see edit. but a treatise on timing of gears and camshafts was not the point of the OP's question. – Solar Mike Dec 09 '20 at 11:17

score 7 · Answer 3 · answered Dec 08 '20 at 12:40

It could be for combination of reasons.

lubrication: the holes will both pump and let the lubricant pass through.
Reduced angular momentum reduces backlash and adds to gearbox responsiveness.
Lighter gears need less shaft support, helping the averall compactness of the gearbox.

score 6 · Answer 4 · answered Dec 09 '20 at 00:57

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Also someone may have to pick the machine up and carry it.
Reducing weight is not just related to the intrinsic function of the machine, its shafts and so on.

answered Dec 09 '20 at 00:57

Adrian Midgley

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That makes sense in two ways: Make a machine easier to carry by reducing weight. Or: A gear could be external, and used as a handle to carry the machine when inactive. – Volker Siegel Dec 09 '20 at 21:30

score 5 · Answer 5 · answered Dec 08 '20 at 12:35

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Depends on the application, but generally there are two reasons why: reduced weight lowers inertia loads (opposite of a flywheel), but more often less material = lower costs if these are high volume parts.

answered Dec 08 '20 at 12:35

jko

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Yep. Particular for gears which are cast and then post-machined, reduced material use is the primary driver. – Jonathan R Swift Dec 08 '20 at 14:03

score 4 · Answer 6 · answered Dec 09 '20 at 07:55

Cost wasn't mentioned yet - Depending on the manufacturer, their processes, the cost of the raw metal, and the accountants it may be that the waste is swept up and reprocessed.

For gears made of brass, bronze, aluminium or titanium this is well worth doing. Any other exotic metals depend on the cost per kilo.

Copper and Lead would be excellent metals for saving too, except they're not used in gears at all being too soft. An electrician would generally save copper wiring extracted from a job, and a plumber/roofer would save lead from roof nails and flashings.

For steel gears, the metal is worth far less. Prices vary but a kilo of brass might be worth $5 but a kilo of steel is only 3c. This is where volumes come in.

Another cost saving is in shipping. A heavy part costs more to ship than a lighter one, or you might fit 6 in a box where only 5 unlightened ones would fit.

Why does a gear have extra holes?

6 Answers6

Motion

Transmitting torque.