Gear box/speed reducers design basics 1

[SShashi]

hi All,

I am new to gear box/speed reducers. would like to explore and learn more on the the industrial gear boxes. while trying to research, i am stuck at sizing of a gearbox. please suggest any whitepapers or helpful docs which can enlighten me on sizing of a gearbox. thanks in advance.

 

[Tmoose]

Are you going to design one, or select one to buy?

Cruising thru a few reducer catalogs might reveal significant ordering criteria, like input and output rpm, max HP rating, continuous and otherwise.
Machine design textbooks often have I mean HAD a chapter on gear design.

 

[BrianE22]

AGMA has several papers on gearbox design.

 

[SShashi]

Thanks guys... i am trying to understand the requirements to deisgn a industrial gear box.. found out that the output torque determines the size of the biggest gear which inturn will guide on the size of the gearbox.. is this right??

 

[BrianPetersen]

Typically, gearbox manufacturers will have various series of gearbox sizes, and yes, each series will usually cover a certain range of output torque. And, it is ALL explained in the manufacturer's catalogs.

Don't forget that there may be a "service factor" that you have to apply on top of the desired torque output, depending on the type of load being transmitted, and that depends on both the input and output conditions. For example, a fan or centrifugal pump is a very steady load, and a reciprocating air compressor is a very unsteady load. On the input side, a 3-phase electric motor driven through a frequency-drive is typically a very steady torque source, and a diesel engine is typically a very irregular source. Again, the manufacturer's selection guide in the catalog explains all this. If your power source contains a mechanical brake and your load contains a lot of inertia ... watch out for braking torques, and be really, really careful about gearboxes that are non-backdriving (typically, worm-gear reducers). High load-side inertia + mechanical brake that "slams on" + non-backdriving gearbox = broken parts!

Around here, the sales representatives for the major gearbox manufacturers are all typically quite helpful and willing to stop in to your shop to help you out. Try that. Call one of them.

 

[SShashi]

Thanks.. that really helps..

 

[tbuelna]

shash1084-

It seems like you are not asking the right question. Normally, if you were tasked with designing a gearbox for an industrial application you would be provided a set of requirements the design must meet. So the question you should be asking is where can I find a good set of guidelines/practices for designing a gearbox that will meet the requirements I was provided?

As BrianE22 noted, AGMA 6013-A06 is a comprehensive reference for designing industrial gear drives and is widely accepted by industry. So purchasing a copy of this document would be a great place to start.

Good luck to you.
Terry

 

[Tmoose]

OP said ".. the requirements to deisgn a industrial gear box.. "
there is a "gear and pulley engineering" group over in the "Mechanical engineers" area

 

[SShashi]

Thanks Tbuelna..

Thanks Tmoose, will post this question in that group..

 

[loki3000]

how is lubrication usually done in a planetary gearbox (if the planets are always in motion, they can't be submerged in oil for example)

 

[BrianPetersen]

No need for the planets to be submerged. As long as they just dip into the oil at the bottom once per revolution, that will carry plenty of oil everywhere inside. And besides, even if they were submerged in oil, what's the problem (aside from more viscous drag at higher revs)?

In automotive applications (automatic transmission), there is an oil pump circulating fluid inside the transmission.

For industrial applications (generally fixed-ratio units), one thing to be careful of is the orientation, because it affects how much oil has to be inside. Horizontal shaft, the planets just have to dip into the oil once per revolution. Vertical shaft is a different matter entirely.

 

[romke]

There are a few aspects in the design of industrial gearboxes that are usually quite different from design criteria for automotive gearboxes. Automotive gearboxes need to be small and light weight. That is achieved in such a way that the gears are designed so that they will last a relatively short time under full load and the life expectancy of individual gearsets may also be quite different. A 5 speed manual gearbox may thus be designed for a total life of say 300.000 km whereas the reverse gear only would last for say 20 km, first gear for 100 km, fifth gear for 250.000 km etc...together totaling the required gear life under the running conditions expected. In German they have a word for that: "Zeitgetriebe" - gearboxes designed to work for a specific time (and a rather limited one, compared to industrial applications).

Industrial gearboxes are designed to be able to transmit a certain torque for a very long time (up to over 30 years....) - and thus the loading of the gears is chosen to be much lower and the size much larger. Eventually the bearings will fail after prolonged use and can be replaced at relatively little cost, where the gears will still be ok.

This difference in design philosophy also has influence on the type of lubricant chosen. Industrial gear lubricants may contain antioxidants and zincbased antiwear additives, but they seldom contain typical EP-additives based on sulphur/phosphorous chemistry as used in automotive applications. The reason for that is that although they do prevent scuffing of gears that comes at the cost of relative high wear. In automotive applications that is accepted because of the weight and size restrictions. With industrial gears the loading usually is quite a bit lower and thus the use of "agressive" additives is not needed and is avoided because most times protection against scuffing is not needed because of the lower loads.

 

[tbuelna]

Good point romke. I would also add that just because a gearbox is intended for an "industrial" application does not necessarily mean it is not high-performance. I have seen some gearboxes used with large industrial gas turbine engines which had gears and bearings that were higher performance and higher quality than any aircraft application.

The approach to oil lubrication of epicyclic gear drive components can vary depending on the specific design. For example, it would not be acceptable to rely on splash lubrication with a high-speed, high-power epicyclic gear drive. The oil windage and churning losses would be significant, there would likely not be adequate oil cooling of the sun gear and the sector of the ring gear above the oil level, and there would likely not be adequate oil cooling of the highly loaded rotating planet bearings. A high-performance epicyclic gear drive usually requires a regulated pressure oil feed jetted to each bearing and gear tooth surface to ensure adequate cooling of these components. On the other hand, splash lubrication would be acceptable for a lower-performance epicyclic gear drive like those used in automotive ATs.

 

[romke]

I would also add that just because a gearbox is intended for an "industrial" application does not necessarily mean it is not high-performance. I have seen some gearboxes used with large industrial gas turbine engines which had gears and bearings that were higher performance and higher quality than any aircraft application.

I did not intent to say industrial gears are not high performance, they are. But "high performance" may mean different things in different situations. When designing a gearbox you have to take various requirements into account (cost, reliability/life expectancy, materials, quality of finish, size, weight etc). The optimal solution in a given situation may vary but they all can be "high performance", be it through low cost, life expectancy, weight or whatever overriding criterion in a given situation.

 

[loki3000]

how are such automotive gearboxes produced? i doubt that every single gear is hobbed like with normal ones (non-massively produced), but i only have some exp with very large gears

 

[BrianPetersen]

Blanks are usually near-net-shape. Then they're machined on production gear-cutting, heat treating, and grinding equipment ... works like hobbing but much faster and generally (nowadays) with CNC control.

 

[tbuelna]

The gears used in automotive ATs are actually of quite good quality, since they must provide good efficiency and low NVH. But these gears are not produced with the same materials and processes used in other industries. Due to the very large production quantities involved in the auto business and the demanding cost pressures, there are very specialized materials, processes and machinery used to produce the gears that would not be practical in any other industry.