Vernier Tech Info Library TIL #2093

Question

Dynamics carts purchased in fall 2009 have high friction.

Answer

A manufacturing problem may affect a small subset of carts we shipped in September and October of 2009, resulting in slightly higher friction that normal. The problem is subtle, but can be detected by observing a freely spinning wheel.

Most carts are fine and exhibit low friction, but since the whole point of the Vernier carts is to offer very low-friction rolling, we wanted to alert you to this possible problem.

To test your carts, please do the following:

1) Hold a cart in one hand, wheels up, in a brightly lighted location.
2) Flick a wheel with one finger to make it spin as fast as you can.
3) Observe how long the wheel continues to spin after the flick. It is sometimes hard to see if the wheel is still spinning, but if you look at the bearing area on the inside of the wheel, you can see parts moving.

If a wheel consistently stops before 2 seconds, the bearings are pinched, and the wheel needs to be replaced. Often the bad bearings will cause the wheel to stop suddenly, rather than smoothly slowing.

It is up to you how we replace the wheels. We can send you replacement axles and wheels, and you can swap them out, or you can send your carts back to us for replacement.

Instructions for field replacment of Vernier CART-S and CART-P axles

Even if only one wheel is sticky, you must replace the entire axle/wheel assembly.

To replace an axle, perform the following steps. You'll need the 3/32 inch allen wrench included with your cart, and will probably want to use a small flat-bladed screwdriver.

-(plunger carts on plunger end only) Release the plunger so the spring is not under tension.

-Use the 3/32 inch allen wrench to remove all four black screws from the cart end cap.

-Pull the end cap off the cart. If there is a plunger, pull it out of the center spring channel, leaving the spring in the channel.

-Push down on the brass axle with your thumb to take tension off of the two silver pins that keep the axle in place.

-Remove the pins, keeping pressure on the axle. You may need to pry the pins out with a small screwdriver.

-Gently release pressure on the axle so that the springs remain in place.

-Remove and discard the bad axle.

-Insert the new axle.

-Push down on the brass axle and replace the two silver pins.

-(plunger carts only) Insert the plunger into the center spring channel, through the middle of the coil spring.

-Attach the plastic end cap by tightening the two outside screws.

-Tighten the two inner screws that hold the teardrop parts in place.

Test the axle to see that neither wheel rubs against the green cart frame. If one does, center the wheels in the frame by centering the axle. Retest.

Created by: gastineau on November 07 2009
Last updated by: gastineau on November 23 2009