In 2009 I bought a Nordictrack elliptical exercise machine, model Audiostrider 990 (PDF link). As home exercise machines go, it's fine. It's uses variable magnetic resistance and it folds up fairly compactly. It also has two front wheels that it can be tilted on them and moved around.
The wheel design of the model I bought is flawed, however. Each wheel is made of somewhat soft plastic and rides on a steel axle, without bearings, and which has an hexagonal nut at the end. Over time, the nut and axle literally pulverize the wheels, as you'll see in the pictures below.
Rather than spend $25 on replacement wheels from Nordictrack, I bought replacements from McMaster.com.
Incidentally, I noticed that Nordictrack has updated the design of the AudioStrider 990 and now use a much smarter wheel and rolling system, perhaps because of complaints from users of the model I own.
The repair was simple, with the hardest part finding a wheel that would be just the right size and not too expensive.
After a bit of searching, I found the perfect set of wheels on McMaster-Carr (a simply awesome website).
Description: Econo-Lite Polypropylene Wheel, Black, 4" X 1-1/4", 1/2" Axle, 275# Capacity
Price: $2.35 each
In order to keep the wheels I bought from rubbing on the outer cylinder, surrounding the axle, I placed 4 washers at the "front" of the axle, and another between the new wheel and the outer nut. The extra washer kept the wheels from moving laterally along the axle.
The lubricant I used is called Sil-Glyde Lube Compound and works great. It's primarily for car and boat maintenance, but I've found many other uses such as lubricating wooden door hinges.
Pictures of the repair
Original wheel, before removal.
Left wheel, showing how the axle destroyed the wheel from within.
Right wheel removed, showing plastic dust from the axle grinding away inside the wheel.
Right wheel, showing damage inside.
Washers keep the new wheel from rubbing against the outer metal cylinder of the axle, and automotive grease keeps the axle lubricated to prevent binding and wear.
Completed repair, with outer washer to keep the wheel from moving laterally and the hexagonal nut holding everything in place.