Does this type of hypoid gearing exist already?

[geesamand]

I am looking at a right-angle gearing application. Horizontal output shaft, input coming vertically from a 1800rpm motor mounted on top of the drive. Historically it's a worm drive or borrowed hypoid or spiral bevel gears from the automotive industry. The trouble as I see it is that the gear mesh is above the centerline and causes either sealing or lubrication challenges.

Here's my idea, and if it's possible I'd like to know if something like it exists already. I propose to use a hypoid gear but mesh below the centerline of the output shaft. The pinion shaft would be supported by a bearing on both sides of the mesh. To give you an idea, I'm thinking about 1800rpm input, sizes in the 5-75hp range, all about 4:1 reduction.

Does this ring any bells? Is there a reason this type of layout is not common?

David

 

[pontiacjack]

Race car quick-change rearends contain a horizontal shaft running from the driveshaft yoke through the differential housing to a pair of splined spur gears at the rear, with a removable plate for "quick" gearing changes. The driven spur gear's shaft runs forward to the hypoid 'pinion' gear.

 

[pontiacjack]

Sorry- forgot to say that the hypoid 'pinion' is behind the axles in the quick-change rears, so it could be "stood up" with minimal lubrication concerns.

 

[pontiacjack]

Wow- I can't seem to get it right... (wish we could edit our replies here).
I misspoke about hypoid gearing- quick-change rears use either bevel or spiral-bevel gears.

 

[geesamand]

So you're saying then that there are not hypoids in racing transaxles?

(Since hypoids lose a few percent vs. spiral bevel, I would be surprised to see them in use in racing gearboxes)

 

[patprimmer]

What about an outboard leg from a boat.

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Pat
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[geesamand]

Interesting idea, but all of the outboard final drives I've seen use a standard bevel set with the mesh above the centerline.

David

 

[tbuelna]

geesamand,

Hypoid gear sets are commonly used in race car final drives. Look at NASCAR or NHRA. The difference in mechanical losses between a hypoid geometry and a spiral bevel geometry of equal power capacity and ratio are not a "few percent". In fact, the difference is probably only a fraction of a percent.

Hypoids and spiral bevels are preferred over straight bevels, for non-parallel axis gear meshes, due to the better face contact ratio characteristics they have. This leads to lower dynamic tooth loads at high PL velocities, and smoother, quieter operation.

As for shaft sealing issues, the oil level in your gear housing should be below the spinning gear teeth if possible. Thus the shaft seals themselves would also be above the oil level. The hydraulic losses and churning resulting from the gear teeth splashing through the liquid oil can be significant, and will also result in heating of the lubricant. A controlled spray of lubricant directed radially into the gear tooth spaces, at both the into-mesh (1/4 of flow) and out-of-mesh points (3/4 of flow) is optimum. If a pressure fed system is not practical, then you might try some sort of slinger disc and scupper arrangement that directs the captured oil down into the gear mesh point.

Good luck.
Terry

 

[pontiacjack]

David- No, I didn't say that hypoid gears are not used in racing. I was suggesting specifically a "quick change" style rearend to possibly fit your vertical-input-shaft scenario. All the quick-change rears I'm familiar with have the pinion centerline in the same plane as the asle centerlines, thus have no need for hypoid gears. (I don't claim to know of every quick-change on the market, so there may be some that position the pinion shaft differently and thus need hypoid gearing).

Some clarification: a hypoid gearset is actually a special case of a spiral-bevel set in which the centerlines of the drive and driven gears do not lie in the same plane. And for historical-trivia's sake: hypoid gearing appeared in vehicles for one (and only one) reason- to allow lower floors.

Terry- you recommend to not have the gears running in the lubricant. But practically every vehicle built for the last century has the bottom third or so of its ring gear constantly running in lube. The resultant splashing and heating are not problematic, and are almost trivial to David, with his proposed 450 RPM ring gear.

 

[tbuelna]

pontiacjack,

Yeah, I know most domestic live axel R&P rear end hypoids are splash lubed. And of course, it works OK at a couple hundred RPM. BUT THAT DON"T MAKE IT RIGHT!!!!

Of course, if you've ever seen a rear axel set up for a NASCAR road course, you'll have noted that they are dry-sumped.

 

[geesamand]

We use spiral bevel cuts frequently in our design for right angle gearing, but we are usually going from horizontal input to vertical output which means it's easy to run a dip lube. And of course a spiral bevel has no offset so it must be meshed about the output centerline. Engineering an answer with the spiral bevel is trivial.

We don't have room for an oil pump, and although flingers and channels are possible they won't help in a cold startup situation. I suspect that a highly tacky gear oil might help cope with the mesh being several inches above the oil level. It might be enough keep the gear teeth protected for that first rotation when the mesh is 100% drained. Since my power draw follows RPM³ it might be a non-issue.

With the hypoid idea I would keep my pinion immersed (churning losses being a downside, but at 1800rpm maybe not a killer) and get one bearing down in the oil and no cantilevered support. The reason I suspect a few percent efficiency loss with this hypoid is the high offset and the churning losses. Unlike NASCAR, I would need a lot of offset to pass by my horizontal output shaft.

 

[pontiacjack]

Going back and re-reading your first paragraph, it seems to me that using a normal automotive rearend would work fine.
1) The only sealing issue would be [any] vent on the old "top" of the housing- just vent it from some other spot.
2) I suspect that lube splash from the ring gear will suffice. If not, add paddles or scoops as required.

 

[tbuelna]

geesamand,

Here's a good tech paper on the effects of windage and churning losses in gears:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19850026094_1985026094.pdf

Regards,
Terry