True LC900 Brake Assembly Repair

How to save a $3000 exercise bike using a $12 bearing

Mr. Determined Enthusiast’s mom called the other day saying that she needed to buy a new exercise bike, because her existing one, a True LC900, was making a really loud vibrating noise while pedaling. She had a repair person out to the house who took it apart and said that it needed a new “brake motor”. Which would cost $900 to buy and install. She figured at that price, she might as well get a new bike. People, this is how our landfills get full.

The Determined Enthusiast will not stand for a $3000 bike being thrown away because of a noise. And whatever ecosystem of manufacturer + parts supplier + dealer results in it costing $900 to replace a $12 bearing, does not get rewarded with another $3000 of revenue. Not today anyway.

The bike uses a simple motor/generator assembly as its resistance unit. Made by the Chi Hua company in Taiwan, it’s basically a three-phase permanent-magnet generator and a flywheel. That transformer looking thing on the side is an eddy-current brake, which makes sense, but the generator adds a nice touch — it allows the bike’s electronics to be powered by the rider’s own pedaling. The Determined Enthusiast approves.

Photo credit: Chi Hua

Sure enough, spinning the rotor caused a clicking sound at even intervals. Spin the thing up to a few thousand RPM and that turns into a racket.

Diagnosis

    The noisy bearing was behind the aluminum pulley located in the left picture.  This pulley had a heavy press fit (foreshadowing), and was removed using an OTC bearing splitter and a shop-made puller.

After removing the aluminum outer carrier, the bearing itself was visible:

See it? There’s a crack in the inner race at the 9:00 position.  Yet still, the recalcitrant bearing required a puller to remove (more foreshadowing).

    The bearing is an SKF 6202-2Z/C3, made in Italy.  This is a common bearing with 15mm ID, 35mm OD, and 11mm width.  The 2Z indicates double shielded, and the C3 is a slightly looser-than standard internal clearance, meaning that there’s some slop in the bearing to accommodate either temperature rise (which would close up the clearances), or maybe the misalignment inherent to the sheet metal housing held together by shoulder screws.  A heavy press fit on the shaft would also be likely to take up some of the bearing clearance (yet more foreshadowing).

    Removal of the shields revealed a bearing full of grease in good condition.  After removal from the shaft, the crack closed and the bearing ran relatively smoothly.   I punched out the ball retainer and thus disassembled the bearing, revealing nearly perfect races and balls (other than the crack).

    In operation, this bearing is relatively lightly loaded and operates in a clean indoor environment.  Some heat generation is to be expected with a brake of this type, but it’s not clear how hot the bearings actually get.  This bearing has most of the drive belt tension loading it, but all of this is well below its design spec. So why did the inner race crack?

Going Deep(-ish) on Bearing Fits

Given the heavy pressure required to remove the pulley and bearing, it seemed worth investigating the bearing fit.  Bearing fits are standardized and the choice of fit depends on a number of factors including whether the inner or outer ring rotates, the load, and other factors.  

This is a relatively lightly-loaded application, running in a clean indoor environment and probably not at appreciably elevated temperatures or at particularly high RPM – most likely the standard recommendations will work.  But to go through the exercise:  The wheel weighs about 10 lbs, centered between the two bearings.  The wheel is driven by a 10-rib belt attached to a pulley outboard of one of the bearings – Gates specifies a belt tension of 150-200 lbs for that belt, which seems about right.  You typically pedal at 120 rpm max, and the cranks are geared up by approximately 16x in two stages, for a flywheel speed of about 2000 rpm max.  So far, all of this is well within the spec of a 6202, which can handle 840 lbs static load and up to 20,000 rpm.  There is also a minimum load, below which the balls will skate and slip instead of rolling, but we’re okay on that spec as well according to the nifty SKF Bearing Select tool. 

For a bearing of this size with rotating inner race and normal loading, both SKF website and the Machinery’s Handbook recommends a “j5” fit, which is classified as a Transition fit, meaning that it falls between Interference and Clearance fits.  The Bearing Select tool recommends a “js5” fit, which is 1um tighter (for a 15 mm shaft).  For a 15mm basic dimension, the tolerance is -3/+5um (js5: -4/+4um), meaning the shaft OD should be between 14.997 and 15.005mm.  The ID of the inner bearing race also has a tolerance, listed as -8/+0um, meaning that the bearing’s ID should be between 14.992 and 15.000mm.  Combining these, the standard helpfully tells us that the resulting fit is between a -13um interference and a +3um clearance, with -11/+1um being the “probable” fit range.

Using a recently-calibrated Mitutoyo micrometer, I measured the shaft’s bearing seat OD at 15.013um.  At least material condition (i.e. best case), this provides a -13um interference, which (barely) meets the j5 fit spec.  A max material condition however, the worst-case interference is -21um, which is almost twice the maximum allowable interference.  Reviewing the table, only the heaviest fits allow a 13um oversize for a 15mm shaft: k5, m5, m6, and n5.  (k5 allows only 12um, but my measurement could be off by a micron.)  And these heavier fits don’t appear to be used for ball bearings with shafts this size under any loading circumstance.

 Why such a tight fit?  A few possibilities:

  • Manufacturing error – the shaft was ground too large, and/or a fine-sizing operation was either omitted or eliminated to cut cost.  Probably the most likely.
  • Manufacturing compromise – they wanted to make the shaft the same size all the way across, and the wheels needed a certain size for their press and were already made
  • Poor manufacturing tolerances – the shaft grinding process wasn’t well-controlled enough to meet the -3/+5 spec, so the engineers specified a heavy interference to ensure that sufficient interference would still be achieved at LMC.
  • Design error – the engineer accidentally specified the wrong fit.  This doesn’t seem too likely given how well-engineered the rest of the assembly appears to be, and how straightforward this application appears to be.
  • Counterfeit bearings with poor tolerances – leading the engineer to specify a larger shaft to ensure sufficient interference.  Possible, but these are going into expensive products.  No one is going to see the bearings anyway, so why bother with a counterfeit when 6202 bearings are basically commodities?
  • Design compensating for previous failure – this seems possible.  Maybe Chi Hua had experienced shaft or bearing failures due to the bearings moving on the shafts for some reason.  So they called SKF, who told them a heavier interference would be okay for their relatively forgiving loads and environment, and so they spec’d it as such.  While this is possible, I haven’t found any references online to ever needing such a tight fit.

“They say that Shaft is a real bad mother—“

Pressing out the second bearing was much easier than the pulley and the first bearing.  Once it was separated from the other parts, it could be inspected more thoroughly. There’s a long gouge, roughly 180° away from the keyway, where the pulley and first bearing ride:

At first I feared that I had caused this while pressing the shaft out of the wheel.  However, the broken bearing shows marking in this exact area, right where the crack is.:

Further, the aluminum pulley also shows a gouge in the same area, and I pulled that off before doing any other pressing.  The wheel also seemed to have something in its bore in the same position.  Maybe it was casting sand that didn’t make it out, and then gouged up the shaft?  Hard to tell but it seemed like dirt or rust.

So there you have it. The combination of an excessively-heavy press fit, plus the stress riser of the burr on the shaft, then cyclically loaded over years of use, and the race finally gave up and cracked. The balls clicked each time they passed over the crack, which created the noise.

Repair

Mr. Determined Enthusiast is well aware that the right fix for this is to turn a new shaft, grinding it to the correct size and cylindricity before pressing on two new bearings in an ISO 1 clean room, while someone with a clipboard dutifully records the proceedings.

But life intervenes and sometimes things need to just get done. So the fix was simple: the shaft was stoned to remove the worst of the boogered-up nastiness (bringing it closer to the diameter spec in the process), and the whole thing was reassembled with two new bearings, using sane amounts of force on the press thanks to the now-more-correct shaft size and lack of burrs tearing up the other parts. The brake is now whisper quiet again, ready for the onslaught of raw power from Mr. Determined Enthusiast’s 80-year-old mother.

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