Most mechanical dock levelers use snubber mechanisms to lift the lips as they are raised, and they close slowly as shocks absorb the weight of the lip. In many of the Kelley dock levelers, however, a push bar and a yieldable rod are connected to a link assembly so that when the deck is raised the push bar drops down. Then, as someone walks across to the outside of the dock leveler, the plate on the push bar catches on the middle bolt allowing the push bar to raise the lip..
Spring Issues - Unless it is due to an obstruction of some type, uneven movement of your garage door is likely caused by a problem with the spring mechanism. Roll-up doors generally have one center-mounted torsion spring. The tension on torsion springs is great. Therefore, it is necessary to call a professional for any repairs to this type of spring mechanism to avoid serious injury. Because replacement of these springs or cables in a garage door can take at least two or three hours of labor, costs can average $300 or more.
The torsion springs on steel rolling doors work the same way as other torsion springs in the garage door industry. When the door is closed, the spring is wound. The spring is secured to a spring anchor bracket on one end and to the shaft on the other. When the steel rolling door opens, the spring in the barrel supports most of the door weight. The remainder of this weight is lifted by the chain hoist or the operator.