How to Epoxy Cold Weld a Broken Window Regulator Shaft

(Posted 8/14/2011) - The manual window regulator shaft on my driver side door broke last week (the gear-reduction shaft into the regulator assembly broke in two). I was faced with spending $70 to replace the window regulator assembly (plus my labor to remove the old twin rail/cable style window carrier and replace with the new unit). Being an engineer it crossed my mind to look into using an epoxy based cold-weld (liquid metal) product to rejoin the broken regulator shaft pieces instead of doing a system replacement. Here's what I learned.

First, products like JB-Weld Quick will set in about 5-minutes and cure in 5 hours, with a resultant 1800 psi of adhesion to any metal. The average shear strength is about 1000 psi, tensile strength is about 2100 psi.

Remember these numbers are in pounds per square inch of bonded surface area. The shaft I wanted to mend was 9/16ths inch in diameter, made of semi-porous pot-metal, used with hand-crank torques of perhaps 2-5 ft lbs.

I suspected that just bonding the broken surfaces might not provide enough strength to resist torque generated shear (shaft area of about 3.14*1/4*1/4 is approximately .2 square inch, reducing all the above specs by a factor of 5, assuming linearity). Specifically, the bond would break at around 200 lbs shear force.

To calculate the shear force with 60 in lbs of torque on a 9/32" radius shaft, I used this solid shaft torsion formula from Wikipedia on Torsion (mechanics):

  • shear stress at surface of shaft is:
  • (2* T ) / (Pi * r^3)
  • T = torque (in lbs), r is radius of shaft (in)
  • (2*60) / (3.14 * .022) = 120 in lbs / .069 cu in = 1740 psi
The max calculated shear exceeds the shear bond spec'd for the epoxy which means likely failure of the bond without further strengthening.

I decided to strengthen the shaft by drilling a 1/8" diameter hole along the center-line of the broken shaft, 3/8" deep on each piece. I filled these holes with JB-Weld Quick and used a 3/4" x 3/32" fine-thread set-screw as a strengthener, and as an additional surface to increase bonding area. I believe I effectively doubled the surface area bond (in 3 dimensions). More importantly, the hardened steel screw added rigidity back to the shaft (70K psi tensile, 40K psi shear).

I should be safe at normal window cranking torques (under 5 ft lbs). The bond would probably fail if the user applied pressure beyond the window full open or close stops however -- that could certainly exceed 5 ft lbs.

Of course the proof is always in the pudding. After letting the epoxy set up I attached the door crank and tried rolling the window up and down -- seemed to work fine!

One other note -- these cold-weld epoxies will fail in hot temperature situations (400 degrees F range). If you decide to bond something that gets hot keep that in mind.

Post-Script: The handle recently broke off at a car-wash as the window was rolled up. So, the moral of the story is these epoxy glues don't handle torsional/shear forces nearly so well as longitudinal forces.

I suspect that I never got anywhere near the ideal shear-bonding that the epoxy can theoretically support (but impulse torque force can be pretty high when you reach a rail-stop on the window while turning the crank, so who knows).

I went on-line and ordered a new manual driver side window regulator ($64 with shipping).