How hot is “hot” on wheel bearings?

[Deleted User]

I have the laser heat monitoring device to check my wheel bearings but how hot is “hot”? If they get too hot, besides letting them cool down, then what? Does that mean you have to grease your bearings before moving? I’m afraid one day I’ll get a hot reading in the middle of a trip and have no idea what to do. 

WilliamA

That's a really good question!  Many trailer folks worry about that without understanding what "hot" is.  The answer to that depends upon the source of the heat. First, a wheel bearing should never be so hot that you can't put your hand against the hub face, or grease cap. It's probable that you have a stamped hubcap in the center of the wheel that prevents you from even accessing the hub directly. That larg-ish cap is part of the wheel assembly, not the hub itself.  The bearing hub is cast iron (steel) with a small stamped steel cap on the end. Assuming you can't actually feel the hub, put your hand on the wheel face in the center. If you can hold your hand on it, it's okay. If it uncomfortably warm to the touch at the wheel, the hub is too hot. 
90% of heat induced failures in hubs comes from hot brakes, not bearings.  Keep in mind that the metal parts of a bearing set would be perfectly happy and function just fine at temperatures that would sear the skin off your hand.  The. Problem (and answer to your question) lies in the boiling point of the grease within the hub. Once that happens, the pressure causes the wheel seal to fail and quickly empty the hub of its lubricant.  I have seen that many times. 
What to do with a hot hub?
1. Take a break. Get off the road and let the hub cool down for 15 minutes or so.  
2. Dial back your brake controller. That's probably where the heat originates...not the bearings.
3. Check behind the wheel to see if you can see grease around the brake drum that has escaped the hub. (It will probably be obvious, as in, a sloppy ring of grease thrown off the drum covering the back of the tire, the frame, the wheel etc.)
If the hub cools down quickly, there's no grease-ring on the back and the wheel feels tight when pushing/pulling on the top of the tire, you can try to move along cautiously for a bit. After a few miles, check it again. 
When wheel bearings fail mechanically (something actually breaks inside the bearing) it happens rapidly and catastrophically, almost always resulting in what's called a "spun bearing" where the bearing race itself starts to spin against the axle. That generates heat very quickly (within a mile or less) up to the melting point of metal with lots of smoke and noise.  Thankfully, that seldom happens. The usual scenario is you are coming down Wolf Creek Pass with your foot on the brake. The brakes heat up the grease to the point that it loses viscosity, boils and blows the wheel seal. Once the grease escapes, things mechanical go south quickly. 
OR:
You have a brake "dragging"...as in, the adjustment is too tight or a brake shoe is "stuck" and doesn't retract away from the drum when the brakes are released. Different scenario, same result. 
The answer to your question lies in the boiling point of the grease.  If you can comfortably lay your hand on the hub/wheel face. You're fine.  I check my hubs every time I go out.  Drive a few miles, pull over and feel the hubs.  If they pass the "touch" test, check them each time you pull in for fuel.  It's also common (on my trailer at least) for one side to be warmer than the other. As long as they aren't wildly different, they're probably fine.  
Tapered (wheel) bearings are rugged and can take a lot of abuse. What they can't stand for long is a loss of grease. Do the "feel" test, watch for thrown grease and when in doubt, back off on the brake controller a bit...

WilliamA 

targ

+1 on what @WilliamA said.

Also, IMO, "hot" in this context is essentially what's referred to as "max touch temperature" and is around 130-140 F.  So if you'd prefer not to lay a hand on a hot surface, you can use those values when checking with your laser thermometer.

ChanW

This info is a great help. Thank you @WilliamA.

I'm always in doubt when I adjust my brakes and the 'spin the wheel' test shows a touch of a slowdown at one point on the circumference. So I adjust again, but it seems there's always a spot that touches.

When on the road, I check the drums for heat (poke through the wheel 'spokes'), after normal braking, and after a hard brake. Never are they hot, maybe a bit warm, so I figure I'm safe with the brakes (and then the doubt kicks in - are they working at all?!?)

The bearings have been a concern because the outer hub cover (decorative chrome) was pretty warm (not too warm per your description though). They never got any warmer, however, and I never saw any leakage, so I had to figure the warmth was 'normal', though I don't understand where the heat would come from in a properly lubed bearing...

tabiphile

Anyone who regularly tows or runs equipment gets into the habit of checking bearings regularly. Whenever you stop, do a quick walkaround and place your hand (carefully) over the bearing hub and get a feel for what is normal. If you plan to use a temperature sensor take a reading and start to get and idea of the norms. After a few hours on the road on a hot day a good bearing will feel a bit warm to the touch, but not hot. On a cooler day, it may actually feel a bit cool. You'll get the hang of it. As has been noted, brakes are more likely to kick up the temperatures if they are even slightly out of adjustment or if they've been used hard recently. This is why it suggested that you service your brakes when you lube the bearings.
Unless you see/smell bearing lube, give the wheels a rest and then move on stopping after a while to see if things are pointing towards a problem. You'll know when there is trouble brewing...you'll feel like you need to pull your hand away to keep it from getting burned.... get your sensor out and measure that!

pthomas745

I remember my favorite Volkswagen reading "How to Keep Your Volkswagen Alive" back in the day used to have the same type of advice for checking the status of wheel bearings that WilliamA points out.

tabiphile

pthomas745 said:

I remember my favorite Volkswagen reading "How to Keep Your Volkswagen Alive" back in the day used to have the same type of advice for checking the status of wheel bearings that WilliamA points out.

yes...after first rolling a cigarette and standing back to take it all in to arrive at the correct frame on mind. 

[Deleted User]

This is great info. I’m starting to understand why they say never pull your camper more than a few hours at a time (3.5-4 is what seems to be the max recommended). I suppose that pulling too long just contributes to increasing your chances of overheating your bearings. 

jgram2

LOL-no chance of towing that long with my bladder! (Did I just say that out loud?) I have fun checking all tires with our laser, nodding knowingly at each wheel. So far, nothing over 115’ with fairly equal temps although the T@B tires run a little hotter, or was it the 4runner? It’s good to see numbers, recognizing speed, load, driving conditions and ambient temps will make a difference.