Curing a Case of the Shimmies: How to Diagnose and Treat Caster Flutter

the /shimm-ies/
Noun
A disorder in which a cart begins a shaking motion, moving back and forth as a result of a problem with its casters
Synonyms: fishtailing, fluttering

You know what we’re talking about. When your cart begins that obnoxious shaking as you’re pushing it through the warehouse. And it’s so loud that your co-worker a hundred feet down can tell you’re coming. You keep your head down because you know if you look up, ten pairs of eyes will be staring at you as you complete your walk of shame. Yes, you have a serious case of the shimmies, and we’re going to tell you how to cure it.

How do I diagnose the shimmies?

Typically, the shimmies are caught in towing situations.  Picture this scenario: You have a group of 6 carts that are all interlocked. You attach your tow-motor to the first cart and begin your journey.  Everything is going well and fine at first. You’re driving, and your carts are tracking. Wheels are rolling without any vibration or shaking. You gradually build up speed. And then, all of a sudden, you can tell something is off. You turn your head and you see it – all of your swivel casters are flopping left and right, and your carts are fishtailing like a car going 80 down an icy highway. You’ve caught the shimmies.

What causes the shimmies?

Swivel Lead Descriptor
Image 1

Before we can treat the problem, we first have to understand its cause. There could be a couple of different sources of this shaking motion, but many times, it’s due to the caster’s swivel lead (or lack thereof).

Swivel lead is the horizontal distance between the center of the swivel section and the center of the wheel. You can see an example of this is in Image 1. There are a few different types of lead:

  1. The first is no lead or zero lead. This is when a rigid fork (the part that holds the wheel and axle in place) is used. A rigid fork is vertically aligned with the center of the caster, making it straight up and down. Therefore there is no distance between the center of the swivel and the center of the wheel hence the name zero lead. You can see a picture of this in Image 2 (below).
  2. The second type is called a standard lead. This is when the fork is slightly offset, creating an angle between the center of the swivel and the center of the wheel.
  3. The third type is called an extended lead. If you guessed that this just means a more angled fork, then you are correct. The bigger the angle of the fork, the larger the lead as it increases the distance between the center of the swivel and the center of the wheel.
Image 2

So what does all this have to do with the shimmies? A caster with a shorter lead or zero lead is more prone to forces

that cause it to be pushed sideways, causing the vibration. This is bad enough on one cart, but stack them together in a towing application and before you know it, your train is fishtailing. A longer lead reduces these forces allowing for better tracking (keeping a consistent path) and decreased shimmying.

How can you treat it?

There are always side forces at play on casters. When you have a shorter swivel lead, those side forces don’t have much room to play with and thus you get a caster that shimmies left and right. To relieve the shimmies, extend the swivel lead out. By taking the wheel farther away from the center of the caster, you give the side forces ample space to flow around the wheel. This results in easier pushing and pulling, allowing the caster to be maneuvered into trailing position more smoothly. And just like that, the shimmies disappear.

Is it contagious?

You might be asking yourself why this matters. Who cares if I’ve got a case of the shimmies? We do, and you should too!  First of all, they’re annoying. If it’s not the constant fluttering that gets you, then the obnoxious noise sure will. We’ve all had the experience of getting a “bad” cart at the grocery store. The one where you can barely get it moving, and once you finally do, the caster is constantly fishtailing back and forth while simultaneously exerting a loud enough sound that makes you consider just grabbing a basket…or maybe trying again tomorrow.

Secondly, and more importantly/seriously, the shimmies cause wear and tear on your caster and your personnel. The constant shaking and vibrating might not seem like much at first, but it will drastically shorten the lifespan of the caster. It causes grinding and loss of tracking, and after some time the kingpin will loosen and it will begin to stretch. When this happens, the ball bearings can come undone. You now have caster failure, and you’ll have to bug someone in maintenance to replace the caster.

 

In addition, as mentioned before, an extended lead allows for easier tracking. Since the greatest amount of effort goes into the initial push of the cart, a rigid fork means a more difficult launch. An extended lead decreases strain for those manipulating the carts and increases safety.

Finally, a fishtailing cart means that your products are more likely to fall off, resulting in damaged goods.

See how a small case of the shimmies can quickly turn into a big problem?

What are the side effects of treatment?

As with any treatment, there are side effects. One side effect is decreased load capacity. This is because when you shift the load away from the center of the swivel fork, it puts more downward force (i.e., load) on to the raceways of the swivel fork. To reduce this risk, try to reduce the load weight. You’ll find that everything rolls smoothly. Another way to fix this problem is to look into a more powerful raceway that can handle more weight.

Next steps

Diagnosing the shimmies is rather easy and you probably already know if you have a case of it. The bright side is that it’s completely curable without any long term side effects. Simply identify which carts are the problem and extend their lead out. In most scenarios, the larger the angle of the fork, the more flexibility you have with the cart and the less stress you have on your casters and employees. Keep in mind that while an extended lead may solve your shimmy problem, you’ll need to adjust your load weight to not ruin your casters all together!

If you still need help resolving your case of the shimmies our caster doctors are standing by ready to prescribe the right fix! Simply fill out our contact form on any page or call us at 800-333-3422.