Hydraulic Transport Dollies

Hydraulic Transport Dollies are used extensively in house moving, barge moving, over-the-road heavy haul applications, and in many other applications.  House movers use hydraulic dollies extensively because of their ability to carry tremendous amounts of weight and still compensate for the unevenness of the terrain over which they are being driven.  The need for hydraulic house moving dollies varies somewhat with the size and weight of the house being moved.  If a house is small, perhaps weighing under 100 tons, and is being moved over fairly level ground, hydraulic dollies may be unnecessary.  However, if a house is quite large, heavy, or is being moved over uneven terrain, hydraulic dollies can make the move quite a bit easier.

Hydraulic dollies can have their hydraulics tied together to produce what is know as the "tripod effect."  A three-legged stool is always stable, because three points define a plane. If one leg is lower than the other two, the tips of the legs still define a single plane, and therefore the tripod is stable.  Hydraulic dollies being used under a house have a similar effect.

Let's take a situation where four hydraulic dollies are being used to move a building.  Each dolly has a hydraulic cylinder in the center which carries the load.  When placing the dollies under the house, care is taken to ensure that each dolly carries an approximately equal amount of weight (this procedure would be a whole other topic). The cylinders on the two dollies at the front of the building (dollies 1 & 2) would be plumbed together to create a common pressure situation. The cylinders on the two dollies at the rear of the building (dollies 3 & 4) would be isolated from each other and the front dollies.  Now picture what happens when dolly number 1 drives over a curb.  As the dolly begins to drive over the curb, it begins to rise.  As it begins to rise, it starts to try to take more of the weight of the house; but, because the cylinder is common pressure with dolly number two, instead of the corner of the building lifting, instead hydraulic fluid flows from the cylinder in dolly one into the cylinder in dolly two.  The entire front of the building rises evenly a distance equal to half the height of the curb that dolly number one is driving over.  The building is not racked, just rocked back slightly toward the rear. If one of the rear dollies were to drive over a bump, the building being moved would rock away from that corner, and the front dollies would again compensate.