A car has 4 tires. A table has 4 legs. So naturally one would want to move a heavy load with 4 roller skates.
So what would happen? When you travel over an uneven surface, one skate slips. That isn’t exactly safe when transporting several tons of weight.
A car has a suspension system that keeps all 4 tires connected with the road, even on uneven road surfaces. A table has legs that are permanently attached. So, on uneven floors, the table rocks and the legs separate from the ground.
Your skates however do not have a suspension system and the load moving skates are not attached to the load. So when the load encounters an uneven surface it rocks just like a table. And just like a table leg that lifts off of the ground, one corner of the load may lift off of one skate. In this situation a skate may lose connection to the load and could possibly slip out. In the worst case scenario, a skate will slip all the way out from under the load causing the load to fall to the ground.
When supporting a load with 4 machine skates, often times the skates can become overloaded. Assuming that the load’s center of gravity is perfectly in the middle, then the weight will divide itself evenly across 4 skates. If the load rocks, the weight will lift off of one or more of the skates; this action results in the extra weight being distributed between the remaining skates. The extra weight is now NOT divided equally between the remaining skates. Instead the skate closest to the one(s) that has lost contact will carry the majority of the extra weight. That skate now has to carry up to twice the rated capacity; if that skate does not have a 100% safety factor, it will fail. This action results in a domino effect. As one skate fails, the weight shifts to the next skate which then is overloaded and fails.
For example, assume you have a 10 ton load with a center of gravity in the middle. The load divides itself evenly between the front and the rear so the front weighs 5 tons and the rear weighs 5 tons. On perfectly smooth and level floor you would need a mathematical capacity of 2.5 tons per skate. If the load rocks, you will need a capacity of 5 tons per skate, to allow for a temporary doubling of initially anticipated load weight per skate.
Uneven floor issues become even more amplified if the load is supported by more than 4 skates. If supported by 6 machine skates, the skates in the center are often times the ones getting overloaded first. On uneven ground the load may teeter-totter like a see-saw. For an instance you may lose contact with multiple skates at the same time and completely overload the remaining skates – which, in most cases will be the ones in the center. Imagine a 6-legged or even 8-legged table. Not all legs will be touching the ground on even slightly uneven floors. The same happens to a load in transit, only a few skates will carry the entire load weight.
(Note: In this article we refer to machine skates which riggers and machine movers often assimilate with steel chain rigger skates. The same concepts also apply to any type of machine moving dollies and skates, regardless of wheel type)
Consider that almost no floor is ever perfectly smooth and level. Floors have recesses, cracks, slopes, and expansion joints. Even if the floors appear perfectly level, over a distance of 10 or 20 feet - often the size of a multi-ton machine - there may be small invisible height differences in the floor. Many times you cannot see the rocking of a load. It is not necessary for the load to lift all the way off a skate to be rocking. Many skates have some type of elastic rubber load pad so it appears as if the skate has contact with load. In actuality the skate may only carry part or none of the weight at all.
So what is the solution?
The first option is to find a way to attach the skates to the load so they cannot slip out. In addition each skate will have to have amply safety capacity to absorb any shifting of the load weight.
The second option is transporting the load on just 3 skates. This solution seems unusual and unstable at first but it may often times is the safest. In a so called 3-point support system, (see www.Toolwell.com for such 3-point machine skate systems) one skate carries the front of the load and 2 skates carry the rear. In this configuration, the front skate will need have to have twice the capacity as each of the rear skates. So for example with a 10 tons load, the front machine skates will need to have 5 tons of capacity and each of the rear skates will need 2.5 tons capacity.
For most people that have not seen this configuration it looks unsafe. Consider a 3-legged stool; it sits stable on the ground even if the surface is uneven. The only requirement is that the weight, or the center of gravity, is kept in between the triangle created by the three legs. So, if you sit on the stool you are perfectly safe. Only when you situate your weight towards the edges of the stool will this support falter.
A load like a heavy machine should be perfectly safe, supported by 3 skates provided the center of gravity remains within the 3 points. This type of configuration will prevent the load from rocking. The weight on each skate always remains the same and the weight cannot shift. Furthermore, because the weight constantly presses down on the skates, the skates will be kept in place by the high friction force created from the constant pressure of load and loading platform.
In many applications, the 3-point support system will not work. This system requires a single contact point in the middle under the front leading edge of the load and many machines do not have a bottom frame that can accommodate this set-up. One option here is to span the front with an I-beam or U-channel beam to create such a single contact point. Short of using this set-up, many machines can only accommodate the 4 skates at each corner. This means that many riggers and machine movers have a 4 and 3 point system in their tool bag to accommodate various loads.