High-performance winches as safety risk: Underdimensioned mechanical design now measurable. Actual measurements at installed systems confi rm considerably higher loads than previously assumed.
As already proven for test installations, an emergency stop at higher velocities and with higher loads can cause forces which exceed two times the rated load by far. This is confi rmed by recent measurements at actual upper machinery drives which are also installed in German theaters. This means that there are signifi cant safety risks if the mechanical components are not designed for these peak loads.
Where safety is concerned, the focus has been virtually only on the new Machinery Directive in the past few years. The new probabilistic approach and functional safety achieved by software were almost always in the center of attention. The standard DIN 56950-1:2012-05, which was revised five years ago, however, already reacted to the trend towards more powerful winch drives which move increasingly high loads at increasing velocities. In its new version, the standard specifi es that all design elements which are located in the force fl ow within the system limit of machines have to be designed for at least to times their rated load or for the accident load. In the standard, the danger of signifi cantly higher accident loads is explicitly pointed out for the fi rst time.
Two times the rated load exceeded by up to 75 percent
Over the past years, a vendor’s drive and control specialists systematically measured the actual forces which act on the design in case of a hard emergency stop of highly dynamic drives in extensive tests on test stands and in two theaters. Result: In case of a category zero emergency stop, the values exceeded two times the rated load by up to 75 percent. Category zero applies if the connection between motor and frequency converter breaks abruptly and the electric motor is no longer able to break. Another applicable regulation, DIN EN ISO 13849-2:2013-02, clearly shows that these scenarios cannot be reliably avoided by technical measures in case of non-redundant hardware, for example frequency converters.
The Machinery Directive explicitly demands that the current state-of-the-art in technology must always be used for the design of safety equipment and safetyrelevant systems. Regarding the mechanical load in case of an emergency stop, the new measurement result have further increased the knowledge about the design of stage winches – and thereby changed the current state-of-the-art in technology.
From tests on test stands, it has been known for some time now that the rated load is signifi cantly higher than 200 percent of the rated load during hard emergency stops. For a long time, people pointed out that there were no real data from installed systems. This argument, which has already been weak considering the Machinery Directive, is now no longer valid. Measurements of emergency stops at two upper machineries of theaters yielded highly alarming results. The usual design limit of two times the rated load was already exceeded at half the rated velocity; and the exceedance was so clear that for safety reasons, the engineers refrained from completing tests at full velocity.
The reason why these insights have not been gained before now is that portable and smart measuring devices – the Stage Guard – had to be developed first. The Stage Guard is attached to the load of a winch. The heart of the device is a highly sensitive acceleration sensor which wirelessly transmits the test values. Based on these values, validated software calculates the infl uencing factors of force, velocity, path and impact.
Risk of falling due to brittle fracture
The Stage Guard has already been used in a German theater at a machine winch with a rated load of 600 kg and a rated velocity of 1.2 m/s. In initial tests, a rated load of 600 kg was attached and category 0 emergency stops were triggered during the “lowering” movement. Even at half the rated velocity, the load exceeded the documented design limit. During brake tests or the “lifting” movement, even higher loads may occur. This was, however, not tested for safety reasons. Only after the load was reduced by 25 percent to 450 kg could the winch be stopped at rated velocity within the design limit in an emergency stop. This means that winches endanger people who work under the suspended loads on stage.
Even if the design can withstand an emergency stop of this kind with no visible damage more than once, the materials used may experience structural changes such as embrittlement due to work hardening which eventually lead to falling of the load caused by an abrupt brittle fracture.
Winches retrofittable with Load Limiter
This signifi cantly higher load mainly concerns winches with higher dynamics and velocities which have been increasingly installed in the past few years. All other winches, however, may also have weak mechanical designs, in particular if the dynamic factors used for calculations were too low or were not considered at all. Responsible parties should generally doubt load assumptions. They initially have only one option to mitigate the risks: They should no longer max out the rated load and rated velocity of the winches completely and perform measurements to get an overview of the actual acting forces. Based on the measurement results, they can calculate admissible loads and velocities which can be used safely.
To be able to continue using the machines according to the originally planned travel parameters, components would have to be re-designed and in some cases reinforced or even replaced. An alternative option is to retrofi t the winch with Load Limiter, an autonomous hydraulic damping unit. The unit ensures that in case of an emergency stop, two times the rated load and the maximum acceleration of 2g will not be exceeded at any time. While there are currently no options for systems with electric drives, hydraulic solutions can be easily retrofi t to create smooth braking ramps even in case of emergency stops and signifi cantly reduce the mechanical load in the fi rst place.
Another possibility is to replace the usually used passive logic circuits by new valves with active logics which allow for signifi cantly smoother velocity reduction due to adjustable braking ramps. Thanks to an additional digital monitoring system, they are also suitable for type-examination tested safety circuits. Individually or redundantly, they provide closed and open position monitoring. They prevent abrupt loads in case of an emergency stop and thus protect the mechanics.
The mere passing of a load test with cordoned off stage area can no longer be a suffi cient evaluation criterion for whether the performance of a machine meets the requirements. The responsible engineers and experts require actual load data from category 0 emergency stops instead. Corresponding measuring series should be performed using Stage Guard which is now available. Based on these measurements, the loads and velocities which can still be stopped safely can be determined and used until the design has been modifi ed or the winches have been retrofit.
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