In the 1982 revision of the ANSI B77.1 Standard the braking requirements for detachable and fixed grip aerial lifts were significantly rewritten. This article relates to these lift types, although the general principles also apply to reversible and surface lifts to varying degrees. The changes primarily addressed the consistancies between specifications of the various braking systems and the purpose of each. The result is a more consistent and practical braking standard.
Two important concerns were: the applicability of providing the various braking systems to all lifts and the adequacy of previously mentioned brakes. This resulted in tables 3-1 and 4-2 in the Standard outlining brake requirements for lifts depending upon the amount of potential energy required to be controlled.
Some lifts which have minimal potential energy such that under all loading conditions they will slow to a maintained stop on their own (through the effects of friction of the line and the drive) within the normally required stopping distance. These are self-braking and should not be required to have any brakes other than an emergency brake. For a condition where a failure of a component limits the friction available and further control is necessary during that emergency, the emergency brake is always required. Similarly, where the potential energy is such that the lift will always stop on its own (without ever speeding up), but in a distance exceeding the maximum allowed (nonoverhauling) a service brake is also required to shorten the stopping distance to within the maximum allowed distance.
Overhauling lifts are those which will increase speed on their own under some loading conditions if not controlled by brakes, motor or other devices for speed control. These lifts are the type normally used in the ski industry and are those which were considered in the previous editions of the ANSI Standard. Here brakes are important to prevent a runaway lift and a potential disaster. Thus, the service brake, backstop and emergency brake previously required are maintained as requirements. In addition, a high speed backstop has been added to provide an added security to the system. The drive train backstop must stop the lift before the lift goes backward more than one degree of a bullwheel revolution. This requirement is more and more important as lifts grow in size, because the energy required to stop the reverse rotation of a massive lift can grow exceedingly fast. This energy comes not only from the line, but also from the inertia of the drive machinery. Failure to effect an immediate stop could create enormous impact stresses or excessive braking heat. This concern is also noted for the bullwheel backstop which must stop the lift before it moves back more than three feet.
For lifts used for downhill transportation, but with the potential to run away in the forward direction, a system must be installed which will dissipate the excess energy to maintain the desired lift speed. It is important that the service brake or emergency brake not be used, since the heat developed could make that brake ineffective. (This potential has been well demonstrated in the trucking industry.) Therefore a separate retarding device must be included in the system.
In addition to the need for brakes outlined above, the form of brakes which may be accepted as reliable is also of concern. In this vein, one of the major changes in the Standard is the provision to use other than just springs of weights to apply the brakes. The 1982 Standard allows in addition the use of other approved forms of stored energy. These might include a pressurized cannister of gas of sufficient size and design to act as a source of pressure to apply the brakes. A battery storing electrical energy to apply brakes electromagnetically, while still a source of stored energy, would probably not be approved because the reliability of such a system could be substantially less than a direct mechanical or fluid application. The phrase “approved stored energy” was deliberately kept to allow innovation over existing systems with appropriate control by authorities. Note that the application of the stored energy to the braking system need not be direct, but may be transmitted to the braking surface utilizing appropriately designed mechanical or fluid linkages.
Brake reliability is often affected by the degree of protection the braking system has from weather and other elements. In practice, this is often evidenced on the disc type emergency brake using the bullwheel flange with the majority of the flange exposed to the weather. Snow, ice and even water can significantly reduce the availability of braking capacity. The revision has stated that the braking system must be designed for all forseeable conditions. It no longer specifically calls for protection from weather, but if such protection is not a part of the brake design, the designer must anticipate the potential for ice covering braking surfaces and design accordingly.
In order to insure that braking systems are continually adequate for their purpose, it is important to periodically test them. This has been previously required only for the emergency brake. Now the Standard requires that all brakes are individually tested according to the manufacturer’s instructions. For friction type brakes this is a torque test, usually with a torque wrench applied to some portion of the drive train. On larger lifts or where otherwise convenient, this test might utilize the prime mover driving against an applied brake using calibrated amphere readings to measure torque. On all torque tests, the manufacturer’s procedure should be followed to insure that testing does not in any way overload the system. Typically, a positive backstop (dog or ratchet type) is merely inspected for structural integrity and for proper functioning rather than being load tested to full impact load.
Brake testing frequency is left to the designer, but I recommend that testing at a frequency of once a month during operating periods is reasonable for most installations.
