Is there a bomb in your transmission?
Tom Shelley discovers some intriguing ways of avoiding shocks and bangs in mechanical transmissions, and the consequential damage that results
Is there a bomb in your transmission?
Tom Shelley discovers some intriguing ways of avoiding shocks and bangs
in mechanical transmissions, and the consequential damage that results
Shocks and vibrations can kill both people and transmissions. When the irresistible force meets the immovable object, the resulting reaction force can shatter hydraulic valves, gears, shafts and anything else in its way. And when a vibration approaches a natural resonance frequency, forces can quickly build up to many times normal with an equally catastrophic result.
The only solutions are either to make everything massively over strong, or build in the right amount of ‘give’ to the system. And one way of ensuring ‘give’ is to replace solid drive shafts and gears with elastomeric belts.
Many years ago, the writer became involved in the problems of a high-speed excavating machine called the ‘Earthmill’. Capable of digging up to 1,200 tonnes of material an hour and throwing it onto trucks, it was offered as a way of planing the top surface off asphalt roads prior to re-surfacing. But, when trialled on a disused airfield, it proceeded for a short while and then split open one or other of the hydraulic motors directly attached to, and driving, the cutting head. It was quickly realised that the cause was shock reaction forces produced by the teeth on the cutting head striking stones in the road surface. The shocks were being transmitted back through the drive train and the resulting repeated hammer blows were fatigue failing the motors.
Concluding that the design was unsuitable for this task, the company decided to offer its UK customers a purpose designed Swedish (Dynapac) road planing machine instead. Interestingly, the Swedish engineers who had designed it said that their first model had suffered from the same problem. They solved it by replacing the direct mechanical drive by rubber belts, which had enough elasticity to absorb the shocks and avoid damaging the hydraulic motors.
Elastomeric transmission belts have advanced mightily since. Contitech, for example, has announced a polyurethane belt that can be used to raise and lower the kind of lifts used on the outsides of some hotels. The belt is 3mm thick, 30mm wide and reinforced by steel cords. Five belts can support a cabin weighing 1,000 kg.
The belts make possible drive solutions with narrow bending radii and smaller components and thus do away with the need for separate machine rooms. The complete drive system is installed in the shaft and occupies around 70% less space than a conventional design. The belt also has a service life two to three times that of conventional cables, runs very quietly and requires minimal maintenance. And since lubrication is not necessary, there is no accumulation of oily dust. The same belt can also be used in car washes, forklift trucks, handling equipment and scissor lifts.
Even more challenging is a belt being developed that will permit the integration of an automotive starter and generator into a single electrical machine. This proposed solution involves a multiple V-ribbed belt made of a newly developed EPDM compound, capable of withstanding engine starting torque.
An alternative solution to the Earthmill problem, not available at the time, would be universal joint couplings with pre-loaded elastomer in the drive train. Centa Transmissions is, in fact, testing a new version of its Centax CX-V product. The elastomer element in this design is inverted through 180 degrees and contained in an outer sleeve, thereby protecting the bearings from construction site debris or airborne rail ballast without impeding cooling air flow. A larger axial bearing and improved lubrication also allow high universal joint angles.
The intention behind such products is not so much to absorb shocks as to avoid possible resonance conditions arising from connection to diesel engines. All diesel engines produce torsional vibrations and should engine speed coincide with the natural vibration frequency of the drive train, damaging resonance can occur. Additionally, in this critical resonance situation, a universal joint acts as an undamped spring, combining with the established torsional resonance to magnify the oscillating torque by as much as ten times the output torque of the engine.
If the universal joint incorporates a pre-loaded elastomer element, the dangerous critical resonance value should be shifted below the operating speed of the drive. The damping quality of the elastomer element will accommodate the momentary critical resonance occurring between engine start up and operating speed and also absorb tensile and compressive axial forces coming from the universal joint shaft.
If the overload is non-repetitive, there are a number of different ways of protecting the drive train against damage, each of which offers advantages and disadvantages.
Tsubakimoto offers torque limiters, Torq Gards, and shock relays. Its torque limiters employ friction discs that allow the unit to slip in the event of an overload. Drive recommences when the overload passes, but not usually at the same initial relative positions.
The Torq Gards are based on the ball detente principle and enable the drive to be completely disengaged in the event of an overload. The devices are designed for one position engagement, enabling them to be reset without loss of synchronisation at rotation speeds below 50rpm. Application examples cited by the company include machining centre gripper drives, X-ray scanners, sluice gates, fodder and grain silo covers, cardboard box carriers, snow ploughs, transfer sections of press machines, plastic extruders, car body spraying arms, pumps, torque wrenches, filter presses, heat sealers and various types of conveyor. In the case of grain silos, they are preferable to torque limiters and other friction based devices which might generate sufficient heat to ignite the grain powder.
Shock relays are electronic devices that monitor motor current and break the power supply in the event of an overload. Application examples cited by Tsubakimoto include submerged mud collectors in sewage treatment, where they protect against unexpected encounters with stones and other hard objects. In bucket elevators, they protect against damage caused by buckets catching on a feed chute or encountering unexpected extra material. In crushers, they stop working when encountering an uncrushable object. They are also used to protect grinding wheels against excessive loads which might damage them, in which case very precise setting is needed requiring use of SD series digital devices.
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