Fig 1. The type plate of a motor with the rated parameters

Operating histories show that machines and systems, despite careful design and production, are affected by external factors that can impair their functionality or cause damage, defects, faults, weak points, disruptions and even failure. When damage occurs due to mechanical loads, it becomes necessary to consider the dimensioning of the drive technology in use.

Initially, of course, operating histories provide some orientation. If a machine does not reach its intended power output and/or achieve the desired qualities, it may be due to its drive systems being intended for smaller loads or other performance problems that may exist in the system. If damage actually occurs, the damage patterns contain valuable information.

In the construction of machines and plants, torque and momentary bending are important mechanical load quantities for rotating components, and they allow the reduction of operating costs. Load measurements consist of a static (temporally constant) and a dynamic part (cyclic or fluctuating load). In the case of regular load variations, the amplitude and frequencies of the dynamic part are of interest.

An additional load analysis can be applied to more quickly analyse and communicate the causes of typical damage, as well as to reduce impermissible dynamic loads through knowledge, experience and/or increased precision. Reduced dynamic loads mean that machine components that are subject to complex loads during rolling, friction and impact processes will “run” better and “reward” the owner with a longer service life.

Dimensioning of Drive Technology

Drive technology is designed on the basis of information contractually agreed between the plant or machine manufacturer and the respective component manufacturer. In the case of existing machines, the first source of information concerning loads is the type plate, which contains values such as the rated speeds and rated outputs. Some type plates also include information on torques and maximum permissible application torques.

The application factors and safety factors actually used by the design engineer often can only be determined by carrying out research in catalogues, drawings, safety documentation, operating manuals and even the contracts between the seller and buyer.

The great variety of such application factors in the case of drive technologies is described in Fig. 1 with reference to DIN 3990.

A driven machine operated uniformly in an application with moderate impacts needs to be designed with 50 percent higher drive power. This means not only that more powerful drive components must be used but also that higher costs will be incurred.

With respect to vibrations, the dynamic factor is very important in the dimensioning of drive technology. This factor quantifies the permissible torsional load vibrations. For stationary gear drives, a factor of 1.2 is often used. Quantitatively, this means that 20 percent load variations are permissible – whether they occur randomly or regularly.

When analysing loads, research is also required to determine if the respective machine or system is being operated as intended. For example, if products other than those specified are being processed in a rolling mill, or if more powerful motors are being used, measurements should be conducted to analyse whether the machine or system fulfils the necessary safety requirements to withstand the new loads over its required service life.

Measuring Load Vibrations

Mechanical vibrations are caused by the effects of forces that arise within the system, or that are periodically introduced into the system. Load vibrations are a special form of vibration and permit a dynamic load analysis. Load vibrations can exert torsional, transversal and axial forces in rotating components and lead to premature fatigue.

Other indicators of load vibrations are forces, displacements, deformations, strains and elongations, and require suitable load measurements. This article is limited to torques in rotating shafts. Torque measurements are useful for the following types of applications:

• Boost power and performance

• Describe and monitor the operating and functional behaviour

• Detect critical load conditions and overloads with “before and after” documentation

• Document and analyse torsional vibration on the basis of dynamic torque analyses

• Determine load spectra and monitor load responses

• Analyse the causes of damage, as a valuable troubleshooting tool

Fig 2. View of a 38-m luxury yacht showing the engine room and strain gauge circuit.

Measuring Torque, Some Examples

A 38-metre charter yacht (Figure 2) was unable to reach its desired speed. Starting at a certain load level, secondary vibrations could be felt in the hull. It was suspected that cavitation was occurring and that the propellers were unsuitable for the hull.

Torque measurements were commissioned to analyse the cause of the load vibrations and determine the propeller characteristics. For this purpose, strain gauges were applied on both the port side and starboard side propeller shafts; in addition, two telemetry systems were mounted and a mobile unit was used to measure static and dynamic torques in recording mode. Figure 3 shows the analysed propeller characteristics for the motor output and the torque for both sides. The following conclusions can be drawn:

Both motors operate at a similar load level. Both the port and starboard motors are evenly adjusted. There is no “turbo lag”.

Neither motor reaches its rated out-put, which confirms that the propellers are not suitable for the hull. The propeller characteristics make it possible to determine which alternative propellers would be effective. Whether the number of blades should also be changed is left to the discretion of the propeller manufacturer.

Fig 3. Propeller characteristics of both drives (left: power characteristic; right: torque characteristic).

Detailed analyses showed that the port side achieves a lower output. Analysis of the dynamic torques showed that one blade of the 4-blade propeller on the port side was projecting. This caused additional load vibrations, which at maximum speed even resulted in initial signs of cavitation. The affected blade was the one just after the attached speed marker, which can easily be identified by the “propeller blade pattern” after removing the propeller.

Fig 4. View of the measurement location on the gear side.

In a power plant drive system, inexplicable gear tooth damage occurred in a gearbox. Based on systematic damage analyses, the occurrence of overloads was suspected. Torque measurements were conducted to determine whether the start-up or switching processes in the pole-changing motors were responsible. Figure 4 shows a view of the measurement location near the gearbox.

In the torque signals, three torsional natural frequencies can be identified. A soft starter was retrofitted to reduce the excessive amplitude of the first torsional natural frequency directly after start-up.

According to torsional vibration calculations, a variable speed pump unit was subject to resonance at certain speeds. Are modifications necessary? How exact are the simulations for this new machine? Which amplitudes occur in the application?

Fig 5. Campbell diagram of the torques during progressive start-up of the pump unit.

Systematic torsional vibration analysis was conducted. Torque measurements at different RPMs made it possible to validate the calculated natural frequencies for the specific frequencies. Figure 5 shows an example of this type of Campbell diagram for torques up to 300 Hz. Based on this representation, it was possible to make the load amplitudes of the torsional vibrations available to the calculation engineer to enable him to further adjust the torsional vibration model. These results were also used to specifically hide the torsional vibrations at 48 Hz.

Dr. Edwin Becker

PRÜFTECHNIK

Condition Monitoring GmbH,

edwin.becker@pruftechnik.com