Axial play double taper bearing / trailer axle

[321GO]

Hi Guys,

I have a couple of questions regarding a double taper bearing for trailer axle application.

There are 2 seperate bearings, greas fitted, back to back and seperated via a "shoulder distance" in the hub (this shoulder distance determines therefor the axial play).

My questions:
- this setup requires axial clearance, correct?
- how much should this clearance generally be?

I need to determine the shoulder distance for the hub, for a correct clearance.

p.s. i already checked the SKF site, but this clearance is not mentioned as far as i can see.

Any input highly appreciated.

Thanks in advance.

 

[EngineerTex]

Is my understanding correct that you have a straight shaft and the hub has a dual taper on it? That is, the cross-section of the hub looks like an hourglass? I may be misunderstanding the question, but with two back-to-back taper bearings, there would be two things to consider: 1) the separation distance and 2) the preload.

The separation distance is pretty well determined by your design. There wouldn't be any guideline for determining this, except that you have a fairly limited amount of room to play with in where they have to fit in the hub.

The preload on the bearings would be determined by how tight you torque the castellated retaining nut. In the past, I had always been told to tighten the nut as tight as I can with my fingers - wiggling the hub while doing so and not using a wrench, then back off the nut up to the first gap where I can put the cotter pin into the spindle. In other words, no torqued preload on the bearings.

In this case, the bearings would experience no preload. That may not be the requirement for your application, but even if it is, then the hub would still have the same shape, i.e., an hourglass shape and axial play is determined at assembly, not during machining.

Engineering is not the science behind building. It is the science behind not building.

 

[321GO]

Hi Tex,

this is what i mean(pls copy past in google):
"Paired single row tapered roller bearings"

I want to determine the correct shoulder distance in the hub.

On the site is also a reference to "Axial internal clearance"

I'm not sure how this axial play is measured though.

Simply with cups tight to cones and play symmetrically devided for each side of the bearing?


Thanks in advance.

 

[EngineerTex]

If you go to the SKF "Product Data" link under the "Paired Single Row Tapered Roller Bearings," then click on the mounting arrangement, in the next window that comes up, there's a link for "Axial Internal Clearance," in the upper right corner, which shows the min and max clearances allowed for these bearings.

http://www.skf.com/portal/skf/home/products?maincatalogue=1&lang=en&newlink=1_14_26

Engineering is not the science behind building. It is the science behind not building.

 

[tr1ntx]

EngrTex -

For vehicle wheel bearing applications, I've always been told to finger tighten, then ADVANCE the nut until the cotter pin will go in, never back it off. Advancing it keeps it pushed back against the thread face, whereas backing it off creates a gap due to the clearance distance between threads. (or if not creating a gap, at least relaxing the slight compressive force)

I don't know that this is a hard rule or if there are circumstances where backing it off and allowing a "loose" thread fit is preferred, i.e., no preload whatsoever. Perhaps it is different for different bearing brands. I just know I've seen it highlighted in repair manuals for several different vehicles "Never back off the nut".

 

[321GO]

ntx,

to my knowledge preload is only applicable for certain application where axial clearance is not tolerated, i.e. when a "stiff assembly" is required.

For instance a tool spindle.

http://www.skf.com/portal/skf/home/products?lang=en&maincatalogue=1&newlink=1_0_83

 

[321GO]

I attached the setup for clarification.



1) Most clearances range from 200-300 micrometer as far as i can see. The only problem is that my bearing is not according their standard, so i don't know exactly which range to choose.
How critical are these clearances? Can i take some sort of average? say 0.2 min / 0.25 max?

http://www.skf.com/portal/skf/home/products?maincatalogue=1&lang=en&newlink=1_14_25

2) suppose the clearance is 200-250 mu, is my interpretation than correct(pls see attachment). Their should always be clearance, at least 0.2mm and max. 0.25mm?

 

[Tmoose]

"I just know I've seen it highlighted in repair manuals for several different vehicles "Never back off the nut". "

I think you will find the "never back off" admonition applies to vehicles with previously mentioned cartridge type bearings, where all the components are provided carefully prefitted to achieve a particular preload. Pretty Typically angular contact type bearings. Common on FWD front hubs, but Vehicles not noted for wheel bearing life much beyond 100,000 miles anyhow.

Harley mainshaft used sets of timken bearings with a few thousandths end play set with spacers, because the mainshaft nut had to be torqued hard to retain the drive hub. Replacing the bearings with generic individual components required playing with spacer widths to get the right end play.

When rigidly preloading any ball/roller bearings one thing to fear is exceeding the amount of preload that will create a thermal runaway situation in service. Very stiff tapered roller bearings have a much more sharply defined limit than other bearing types, so if the design is such that things could get a little bit tighter from heat generation, loose is way better than a little too tight.

"In the majority of applications, the operational clearance should be positive, i.e. when in operation, the bearing should have a residual clearance, however slight,........"

http://www.skf.com/portal/skf/home/products?lang=en&maincatalogue=1&newlink=1_0_83

There is theoretical fatigue advantage of running light preload, in part because it ensures a high degree of load sharing with a large load zone, but it is pretty small, anyhow. The radial clearance is a fraction of the axial clearance, so the chance of a few rollers handling the load is moderately slim. At some point shaft and bearing housing deflection or machining inaccuracies will be more dangerous anyhow.

I could provide examples of "back it off 1/6 turn" from factory manuals of a few cars and light pickups from the 50s to the 90s using tapered roller bearings adjusted via axle nut. `

 

[moon161]

End play on the front hubs for my benz is spec'd at .01-.02 mm

 

[tr1ntx]

Tmoose,

Thanks for the explanation, but now I'm confused slightly.

I have a 98 Chev truck and it has tapered roller bearings on the front hubs and the Chilton manual for it says "always tighten, never loosen the nut". My travel trailer manual too. I realize that the writer might have just copied & pasted from another text, not realizing that they were talking about different types of bearings.

In HS I had a VW Karmann Ghia and I once had the brakes done at one of the national chains. Three days later (about 50 miles city traffic) one of the rear wheels came off. The manager of the shop said "well I guess he (the mechanic) backed off the nut instead of tightening it to put in the cotter pin and that let it work back and forth and shear the pin." The pin was sheared, with the middle parts still in the hole.

I don't mean to hijack this thread, but the OP asked about trailer bearings and having recently re-packed my travel trailer's, the issue of back off / tighten stood out to me. Paying attention now, I might see the difference. He's talking about back-to-back bearings with a small space between them, whereas mine has an inboard and an outboard bearing with its own race location machined into the hub, about 4 to 5" apart. Mine taper in and his taper out, I guess, so maybe that is what makes the difference. (Or like I said, maybe my book is wrong. And my shade-tree mechanic friends.)

I might have seen "back it off 1/6 turn" previously on other vehicles, and since forgot, but my truck and trailer manuals say tighten. I drove new trucks for 8 years, then company trucks for 4, so I went a long time without doing any mechanicin'.

 

[htfptm]

tr1ntx, your post brings something to light that is likely a source of confusion between the OP question and most of the responses.

On a pair of opposed tapered roller bearings, like a trailer hub, the axial play is not set by machining the hub or spindle. It is set by tightening the nut. The spindle just needs clearance at the shoulder so the inner race does not bottom out before the nut takes up the play.

On a paired tapered bearing, the end play is built into the bearing. Here, the nut is tightened against the race. I think most front wheel drive cars (or rear wheel in the case of your Ghia) use a pair of ball bearings rather than paired tapered bearings in this application. In either case, axial play is not set by the nut, it's built into the bearing (not the spindle, not the hub). The nut is tight because it is holding the hub onto the splined axle.

Anyhow, when your VW mechanic finger tightened the castle nut on the Ghia rear wheel instead of the 250 ft-lb or so torque it required, that is what doomed it to become 3 legged (ouch!!).

 

[321GO]

What do you guys use a rule of thumb for such axial clearances for dual taper bearings?

Suppose, your dealing with a non-standard or absolete bearing?

Does anybody in the "bearing scene" know why these clearances are so bearing specific on the link below?

http://www.skf.com/portal/skf/home/products?maincatalogue=1&lang=en&newlink=1_14_25

Thank you in advance.

 

[GregLocock]

The big problem with using double tapers for wheel bearings is that the clearances have to account for thermal effects from braking, and often the difference in radial load on he two races, which mean that the optimum setting may not lead to equal life for the two races.

The Timken TRB application notes go into this in some detail.

For a front spindle the nominal assembly condition can be anywhere between slight play and heavy preload, for maximum life depending on the configuration. As such I'd be very wary of any one size fits all recomendation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[juergenwt]

Hand tighten the crown nut until tight and the cotter pin goes in thru the crown slot. Back off one slot.

 

[321GO]

Yes, i now agree a universal value makes no sense. Seems to depend on a lot of factors(different expansion rates of both races/materials used for shaft en hub/operating temperature).

Although i have read that a so called "back to back" arrangement is rather good at canceling out temperature induced clearance adjustments.

All the automotive hubs i know of have this "back to back" setup, and as far as i can tell, all use axial clearance (probably to compansate for the temperature issue).

I looked up a specific manufacturer's repair manual for clearance and as far as i can tell, they use -0,05mm...0,05mm on a dailgauge as range(0.1mm total).

 

[Tmoose]

For a wheel hub, the back to back arrangement provides much greater capacity for moment loading, so I doubt any other arrangement will be found, whether the housing rotates ( RWD front hub) or the or the shaft rotates ( drive axle ). That arrangment is a happy side effect of making an assembly adjustable via a shaft nut.

http://www.skf.com/images/cat/images/1/1_0/0008f28.gif

In a motorized machine tool spindle the rotor heats the shaft and adds to the already normally higher inner ring temperature from bearing generated heat. Process heating or cooling can sway things quite a bit. SO generally in operation the warm shaft and bearing inner race get larger diameter (greater preload) but with a B-B arrangement, the shaft also get longer, tending to relieve the preload. Under real hard working conditions the effect, although helpful, offers limited protection. There is a length that is considered "perfect" because the axial growth would cancel the radial growth if all the rotating components were the same temperature. Interesting concept, but I don't think Even Walt Disney could accomplish that technical magic. Americans will Bring a high speed spindle up to full speed without any kind of temperature-equalizing warm-up sequence. The preload of a fast bearing in a cool, thick spindle housing goes from "I'm OK" to expensive smoking junk in a few seconds, ceramic balls or not.

Here's a link to page 190 of a precision bearing catalog. Figure 14 is a chart of the relative very approximate "speedability" factors of various 15 and 25 degree angular contact bearing arrangements and rigid (non-spring) preloads.

http://www.industryarea.com/Super Precision Bearings-Y1319-S190-catalogue.html

The difference in race offset between light and heavy preload might be about 0.001 inch in spacer length, or 0.0005" per bearing (yes, it varies a lot with bearing type and size and even manufacturer - SKF's "light" is not the same as FAG's)

I suspect that there is a similar chart for immensely stiff tapered roller bearings, but I have not stumbled on it yet.
The speedability is 100% related to bearing heat generation VS heat dissipation. There is no recognition of the worsening of spin-roll ratios at high speeds with low preload (not a concern or applicable to wheel bearings, usually, thankfully). If I Exceed my arrangement's speedability, then the thermal runaway Might get here in the next 2.6 minutes, or wait until 2 hours into the shift, unbut it's headed my way.