safely braking the bridge

Tom Shelley reports on a simple development that allows the safe arrest of very heavy falling objects in a critical application A simple braking system on a shaft is able to safely arrest a falling ferry access ramp in the event of lifting and support system failure, even if the ramp weighs, as some do, up to 700 tonnes. A fail safe locking collar engages the shaft when hydraulic pressure is released, but the even cleverer bit is the way the same unit avoids shock inertial loads, which because of the nature of the load being stopped, would otherwise be massive. Safety is an ever increasing concern of everyone involved in public transport, especially in view of the explosion of no win, no fee litigation cases. Ferry port operators are particularly concerned, especially in the light of past accidents, where gangways, walkways and loading ramps have failed. Current practice, according to Steve Smith, UK Sales Manager Civil Engineering for Bosch Rexroth, is to turn to completely duplicated systems. This has to be done whether the ramps are lifted by hydraulic cylinders, as they are in 70% or cases, winches in 20% of cases, or some other means as are used in the other 10%. Speaking at a seminar at the just held motion and control show, Mr Smith expressed the opinion that such duplicated systems were unnecessarily complicated and expensive, and did not in any case achieve total 100% redundancy. In the event of system failure, whether hydraulic or electric, the operator would be liable to find that they did not have full control. In consequence, Bosch Rexroth has spent the last ten years in consultation with operators, engineers and authorities, and working in conjunction with German brake manufacturer Sitema, has now come up with a most elegant solution. This, named RoSafe, takes the form of a small slave hydraulic cylinder, through with a long piston rod passes, attached to the end of the bridge of ramp. The rod is equipped with position sensing along its length. Inside the cylinder is a tapered collet, which engages on the rod in the event of removal of hydraulic pressure. In the event of failure, and detected movement of the piston rod when it should not be moving, pressure is released, the collet engages the rod and is pulled downwards, and acts as a piston, pushing oil out of the cylinder through a restriction, which thus decelerates it. The system has been thoroughly tested on a 150 tonne test rig, and is now CE marked and approved by the Health and Safety Executive. It is consistently able to stop 150tonnes in 200mm and 0.2s. Now that the tests have been completed in in June this year, Mr Smith is confident that "We are now close to selling one. " The same approach can be used to safety brake any load moving in a straight line and adds to the company's considerable experience and expertise in coming up with ingenious solutions to particular problems. Bosch Rexroth Pointers * In the event of detected rapid movement when none should be taking place, hydraulic pressure is released and the collet engages on the rod. * The collet then acts as a piston, pushing oil out of the cylinder through an orifice, thus decelerating it