Of course, overspeed switches are generally confined to major motorised units. This means that they are often included with the unit as supplied. Whether they remain as efficient as when they were installed, however, with no inspection regime, remains an open question.
Overspeed regulators have been in use for decades, until recently, largely in mechanical formats. As might be expected, electrical units are becoming more common.
There are several different overspeed sensing devices used with hoists. For example, a typical shaft hoist will have a Lilly overspeed to detect if the cage is overspeeding, a motor overspeed device to detect if the motor is overspeeding, and when the hoist incorporates a motor generator (M-G) set, overspeed protection is provided by a M-G set overspeed. A slope hoist, in addition to the above overspeeds, will also have a relay fly weight on the brake car to detect an overspeed condition.
According to John Perkins, of Ingersoll Rand: “Material handling equipment is mono-directionally loaded. Therefore, the power required to lift the load in the up direction is applied to overspeed the motor in the down direction. Most industrial commercial material handling hoists use synchronous speed motors that provide the overspeed protection inherent to the equipment.”
The overspeed device would be commonly considered in safety critical applications in which multiple redundant equipment is required by code or standards, or in applications that utilise a power source without synchronous speed devices.
Ingo Ruehl, of CERN (the European Organisation for Nuclear Research), believes that one reason overspeed units have largely escaped precise regulation is because of the difficulty of accurate inspection. “It would be very difficult to see how you could start an effective testing regime,” he says. “Just think about it, the inspectors would have to see the system in operation to know that it was working, and that would largely be impractical.” The fact that both the US and EU lack precise regulation would seem to bear out this fairly fundamental difficulty. The US, for example, does have model testing, but this is of course not the real thing and, in fact, far from it.
Model testing is not so widespread in Europe, though most crane suppliers claim that overspeed systems are provided with their units.
But if the US does not have enforced government regulations on the use of overspeed units it does have established standards regulating their use. Especially prominent in this regard is the National Electrical Manufacturers Association (NEMA). The standards it publishes provide practical information concerning ratings, construction, test and performance and manufacture of a whole range of industrial control equipment. These standards are used by the whole US electrical industry.
There are a significant number of manufacturers catering for the US market. These include Hubble, Leviton, BWI and others.
Konecranes supplies the so-called ‘e DynALift’ and ‘DynAMove’ overspeed units as separate items. DynALift Vector for hoisting and DynAMove Vector for travelling are both speed control systems. They are based on the latest inverter and vector control technology specially designed for crane applications “with good torque and brake control management, safety features and robust mechanical construction,” it says.
Hubbell industrial controls have a well established profile in the US and provide industrial products for overhead crane, locomotive machines and fire pump controls. Its speed switches are used on a wide range of cranes throughout the country.
Hubbell’s speed responsive switches detect motion, or sense over-speed or under-speed. The company provides both direct coupled or non-contact type switches for directional, non-directional, plugging, anti-plugging, over-speed, under-speed, and also zero-speed applications. Crucially, it provides a range of enclosure styles including surface or flange mounting which match the aforementioned NEMA standards 3, 4, 7, 9, 10 or 13 construction.
Avtron Manufacturing, a Cleveland-based company which specialises in automatic test equipment and automation systems, provides engineered combinations of Avtron encoders and overspeed switches from Hubble, Leviton, and others. These overspeed combinations are ideal for applications in crane, hoist, mines and other safety-critical applications. These units provide overspeed protection and speed feedback from one assembly.
One of the newest devices to emerge is the BWI Eagle ‘Overspeed monitor’. Interestingly, BWI makes much of the fact that this unit is factory programmed to the customer’s specific trip point. This is both a sales and safety feature for BWI for it ensures, according to a spokesman, that the company is able to effectively control the quality of the devices it sells.
Field calibration is a major problem for overspeed devices and the fact that customers will recalibrate as they see fit is another inhibiting factor to establishing an overarching regulatory regime. Of course much the same applies to overload systems, especially for those on smaller units.
As referred to earlier, testing can be a significant challenge in this context and BWI uses a modelling system known as the ‘two-position test switch’, which precisely simulates a true overspeed or fault condition without operator adjustment or recalibration.
Placing the test switch in the overspeed position internally increases the sensor input signal to the preset trip point and causes the control relay to de-energise. Placing the switch in the fault position internally shorts the sensor input signal, which simulates a loss of sensor signal, also causing the control relay to switch off.
In terms of innovation in the overspeed arena, Safwatch from American Crane is a notable product. The big claim for the new switch is that it which exceeds the requirements of NUREG 0554 – a significant point when one realises that NUREG stands for the US Nuclear Regulatory Commission Regulation. Safwatch also exceeds the American Society of Mechanical Engineers NOG-1 Specifications – “2004 Rules for Construction of Overhead and Gantry Cranes (Top Running Bridge, Multiple Girder)” which also pertains to nuclear applications. This is, almost by definition, precision equipment.
It includes independent hard wired watch dog circuit monitoring, SafWatch processors and power supply, while ‘unique encoder line receivers’ provide feedback for hoist motor, drum direction and drum speed. Rack mounted modular input/output units provide flexibility by allowing a mixture of input and output types.
The unit can detect overspeed in excess of 115% of critical lift speed and detect overspeed in excess of 115% of non-critical lift speed. The brakes are automatically applied when an overspeed fault is detected.
According to a spokesmen for the company, “our grading by the nuclear regulatory commission for Safwatch is an indication that this system can be trusted to perform to the highest standards.”
Drive manufacture ABB provides speed monitor and speed matching. The speed monitor function ensures that the crane motor speed remains within safe limits to prevent overspeed. The speed matching function continuously compares the speed reference and the actual motor shaft speed to detect any possible difference. One of these functions will stop the motor immediately if a fault should occur in the operation of the unit.
In conclusion, overspeed is an area of growth and development. There are at least five patents ongoing for new types. One example includes a bidirectional overspeed device comprising a bidirectional governor, a first safety brake, a second safety brake and a bidirectional linkage apparatus. In the event of an overspeed condition in either direction, the bidirectional governor clutches the governor rope, thereby operating the bidirectional linkage apparatus which connects to the governor rope.
Given the fact that overspeed units exist and in such a variety of formats, despite the difficulties, a federal or at least common state standard would make a lot of sense. Granted no one believes overspeed units have any particular reputation for unreliability, but that is no reason for complacency.
The aforementioned model testing, while not perfect, does work so the complications of an inspection regime should not be insuperable. This however does not seem to be on the cards at any time soon.
Avtron Manufacturing, a Cleveland-based company which specialises in automatic test equipment and automation systems, provides engineered combinations of Avtron encoders and overspeed switches from Hubble, Leviton, and others overspeed 5 Ingo Ruehl, of CERN (the European Organization for Nuclear Research), believes that one reason overspeed units have largely escaped precise regulation is because of the difficulty of accurate inspection overspeed 1 Overspeed systems are commonplace at CERN (the European Organization for Nuclear Research) overspeed 2 Testing overspeed systems can be a significant challenge overspeed 3 Another hugely expensive load is carefully lowered at CERN overspeed 4
