Gear Materials

In designing two mating mechanism, consideration should be made either which to comes first to fail. Member or moving parts which is more conveniently made or can be replace readily should be the one to fail first. Factors like material economy and probable equipment downtime should be well considered. Take for example between key and shaft. If your shaft is to fail first than the key. It will not be convenient and cost wise. That is why the key should fail first than the shaft. Now for the worm and gear scenario, i want you think which should fail first, also consider the forces involve between the mating surfaces, there you should find the answer of your question

the same philosophy does not hold good for spur or helical gears with the same gear ratio of 40. i came to the conclusion that since the rotation is both sliding and rolling in worm type arrangement and simply rolling(more or less predominantly) in the other arrangements, may have bearing on material selection. i also don't know for sure, this is the only consideration or some thing else.

This is the main consideration.

Rolling and sliding: if both materials are hard, then more lubrication is required. BUT the wear will be higher on the worm ... which is more costly to replace than the wheel.

Having a softer wheel material permits easier manufacturing of the wheel.

Rolling and sliding: if both materials are hard, then more lubrication is required

But why?

wear will be higher on the worm ... which is more costly to replace than the wheel.

Why worm is more costly as it can be made on inexpansive simple lathe, on other hand you need an expensive hobbing machine to cut wheel. wheel consume more materail and time than of worm so how it can be less expensive? can you explain?

Having a softer wheel material permits easier manufacturing of the wheel.

How much does that effect? any assesment please?

1st Why: If the lubrication is not enough and adequate, ---> 2nd Why.

2nd Why: To prodyce a hard Wheel, is costly because of the rectification Grinding required after hardening...while the worm is easier to rectify...?

All the Whys are related.

You have not replied original question, why more lubrication is need with harder components,

2nd Why: To produce a hard Wheel, is costly because of the rectification Grinding required after hardening...while the worm is easier to rectify...?

Thats not right,

If you use hard wheel it will eat worm in few days thats the real reason.

You are shouting!? Please contribute with moderation and READ the comments properly.

- It is more costly to produce a hard wheel. You have more teeth to grind and therefore more material to remove which makes it a longer and harder operation than to produce a WORM. If you think about it and/or have ever been involved in the making of such parts then you should have agreed that the reason they make the wheel from a softer material than the worm and why the worm is generally of hard material is mainly a process, cost and practicality issue.

You can produce a Worm and Wheel both very hard and it will work perfectly well if you can avoid galling and pay for it (everything is relative)...

So, don't be too patronising and remove the SHOUTING from your comments. After all, you are free for your opinions.

Rolling and sliding: if both materials are hard, then more lubrication is required. BUT the wear will be higher on the worm ... which is more costly to replace than the wheel.

I think the softness is to some extent, coincidental. The bronzes used for gears can be quite hard but still very useful in worm gearing (where there is a lot of sliding friction) because the coefficient of friction between steel and bronze is so low, and because the likelihood of galling is low. (This is not unlike the reasoning for using bronze nuts on stainless steel threads to avoid galling.)

Two soft steel gears running together require as much lubrication as hardened steel gears.

Having a softer wheel material permits easier manufacturing of the wheel.

But if this were the case, then the worm, which is more difficult to machine, would be the bronze candidate. Bronze has excellent machinability properties, but most gearing is not bronze, due largely to cost issues. Worm gearing is (more or less) unique in having two distinctly different metals in mesh. Worm gearing is unique in having a very large sliding friction component. Therefore, it seems likely that the material selection relates to friction.

Well, the conclusion is that both friction and cost play a role. You can make a Bronze Worm and a Steel whell if you like since the friction criteria will be the same, but the worm will wear quicker than if it was the other way round.

i like the galling issue which I forgot ... thanks

In most any gears it is common practice to use an inner material which is more ductile, less brittle, and quite tough, while the outer layer is hardened, more brittle material for longer wear. The combination of both materials will produce a gear that is both tough, and hard, which will not crack and shatter, or wear prematurely.

With worm gears I've always worked with Radicon units. The crown gear was always phosphor bronze while the "worm" would be steel. The crown would be mounted on a steel core in all but the very smallest units.

Just found out Radicon is no longer a UK company, I'll hold my tongue as to the new owners!

It should also be noted that in this arrangement - Steel worm, bronze Crown the choice of lubricating oil is critical - the wrong oil will cause rapid wear and subsequent failure of the crown wheel.

The oil is quite different to that used spur gear arrangement.

Minimum wear occurs with two sliding surfaces when the ratio of hardnesses is 4:1, refer Tabor, Friction and Lubrication, Methuen.

When you refurbish kitchen drawers, glue a narrow strip of laminex or formica or melamine sheet to the cabinet runners. Plane the drawer slides, and reinsert. They will slide a treat for a lot more years.

One car company found out that having chrome-plated bores with chrome rings was a no-no.

Very interesting,under which conditions does Tabor establish this ratio? Which wear is concerned ? According to other documents other values are indicated even zero wear i.e. a wear which is less surface roughness (IBM research).

I do not know if you noticed but we try here to share the knowledge. May I ask you again about the value you mentioned :

under which conditions (tribological) was obtained by Dr. Tabor the result you mentioned ?

DEAR Mr. LV PRASAD,

The Idea is to have more or less equal ware, on Pinion and Gear. For Low Speed Applications,for example, if the Gear has a Hardness of 230 to 240 BHN, the Pinion will have a Hardness of 270 to 280 BHN.

The Pinion teeth will frequently will come in to contact than the Gear Teeth and the wear will be more for Pinion and to off-set this effct the Hardness of Pinion will be more.

Thanks,

DHAYANANDHAN.S.

I think so: 1) wormgears are continuosly sliding and the ideal material is some kind of bronz and in high volume production and low comitment i'd choose this combination for easier matching parts;2) The "both 8620" requires extra work for matching those parts in the workshop, and not simply working in the customer's machine, this is only avoidable for a few gears manufaturers :Always you need that extra work and anyone can make those bronz wormgears and always work without that extra work.-