Has this ever happened to you? You’re deeply involved in some project or task. As you pick up an object to carry it across the room and out the door, you realize that the object is too heavy, or too bulky and awkward to handle easily. Your eyes scan the room, looking for something with wheels or casters – you see an office chair, a small dolly under a potted plant, a plastic rolling cart with a printer on it.
If you select one of those wheeled devices, place your object on it, and try to wheel it away, chances are that things may not go quite as smoothly as you had hoped. You may encounter a range of problems:
- Two of the wheels become stuck when they hit the threshold as you go through a door, and the object you’re moving almost topples over.
- One of the wheels bumps into a small piece of debris on the floor and comes to a quick halt, causing the rest of the wheels to pivot sharply around the obstruction.
- You move from smooth concrete to a carpeted floor – and you have to push three times harder to keep things moving along.
- You pause briefly to rest, let go of your load – but rather than stopping, it keeps rolling along on its own.
- The load feels extremely unstable, and you’re afraid the whole thing might topple over.
Even though these may seem unrelated, they could all be caused by the same problem: wheels that are the wrong size for the task. Selecting a caster’s wheel diameter is one of the most overlooked areas of choosing casters.
In this article, we’ll look at the prime suspect for the problems above – caster size – and see how the right size caster can make material handling tasks easier and safer; and how choosing the wrong wheel size can do just the opposite.
Larger Casters Roll over Obstacles
In a perfect world, with smooth, clean, even floors, wheel size is not as important as other caster characteristics when it comes to determining performance. However, in the real world, floors are very rarely perfect.
Uneven Surfaces – Indoors, floors often have cracks, thresholds at door openings, or adjacent surfaces that are not quite even with each other, such as those found at expansion joints. Outdoors, the problem of uneven pavement poses an even larger problem, as grates, potholes, bumps and other problems come into play.
Each of these uneven features present themselves as an obstacle for the caster. To keep moving, the caster must roll up and over the obstacle. For small diameter wheels, the force required to push the caster up and over the obstacle can be quite large, because the vertical component of the caster’s motion, when compared to the horizontal component of motion, is quite significant. For large diameter wheels, the obstacle will present much less of a barrier. The caster will seem to bridge the obstacle very easily, and keep rolling with much less force required.
Debris on Surfaces – Even if indoor flooring or outdoor pavement is perfectly smooth, debris can be present. When it is, it can be a barrier to the caster’s forward motion. If we consider a 2-inch caster, a pebble as small as 1/4 of an inch represents a barrier that is 25% of the caster’s radius. This is a significant obstacle for the small caster to climb over.
The same pebble, when in the path of a 6-inch caster, is only 6% of the caster’s radius. To this larger caster, the pebble looks more like a grain of sand, and can be easily rolled over.
Soft Surfaces – When a caster wheel presses into a soft surface, such as a carpeted floor, the surface will form a depression around the wheel. The caster, in order to move forward, must roll up and out of the depression. Larger wheels have two advantages in this situation: they form much less of a depression, due to the fact that the weight of the load is spread out over a larger area; and it is easier for the wheel to roll over the depression, in the same way that it was easier for larger wheels to roll over debris, as described above.
As a general conclusion, then, we see that in each of these circumstances – uneven, soft or debris covered surfaces – larger diameter wheels will perform better than smaller diameter wheels.
When Smaller Casters Have the Advantage
The equations of motion for casters show that the force required to push the caster goes down as wheel diameter increases. In other words: the bigger the wheel, the easier it is to push the caster forward.
This may not always be what you want. For example, office chairs equipped with casters must be able to roll – but with some resistance to forward motion. If they rolled too freely, they might roll away just as someone is trying to sit down in the chair, which could be dangerous. In this case, choosing smaller casters will increase the force required for motion, which will help the chair to stay in place.
Another problem with large casters is that as wheel diameter increases, the load is raised too, which elevates the center of gravity. For stability and to avoid tipping over, a lower center of gravity is better. Medical carts in hospitals, for example, often have smaller casters. This helps keep them from tipping.
What’s the Ideal Wheel Size? Call Douglas Equipment for the Best Answer!
Wheel size is just one of many characteristics that determine a caster’s performance. Things like wheel material, tread width and shape, bearing style and many others are important, too. To find out which combination of variables are right for your situation, please give the experts at Douglas Equipment’s customer service department a call. You can reach us at 800-451-0030 (or 305-888-3700, if you are in the Miami area). Contact us today! We hope to hear from you soon!
