Good design practice for drives extends life and reduce costs

Good design practice for drives extends life and reduce costs       

   
Machine makers today have to design for increasing cycle rates, higher operating times and perhaps fully automatic 24 hour operation.  Ambient conditions and increased powers demand more effective cooling.  The machine designer has to balance the warranty offered by the drive supplier, which may be two years excluding wearing parts, against increasing customer demands for reduced Life Cycle Costs (LCC) and Total Cost of Ownership (TCO).  How can drive products such as motors, brakes, gearboxes and frequency inverters achieve longer life under conditions of increased usage?  Lenze have applied design and sizing expertise to these products and achieved this target.

Electric motors are seeing a trend to increase operating speeds under inverter control.  What was a running speed based on 50Hz frequency can now be 70Hz, or even 87Hz with the corresponding increase in power and heating.  One of the weak points is the shaft seal between the motor and the gearbox.  Through a joint project with the seal manufacturers, Lenze have optimised this seal achieving lower surface pressure and a life increase that can be from 3000 to 9000 hours.

Motor costs can be reduced in other ways.  With servo motors Lenze can supply pre-assembled plugs and cables for the power, feedback and blower connections.  This speeds built times and any subsequent maintenance operations, furthermore a qualified electrician is not needed.  For standard motors used in frequency inverter operation, it is possible to choose single or multi-turn encoders as a standard motor option.  This also keeps a lid on costs.

Spring applied motor brakes used for holding and emergency stops can be a weak link when cycle rates increase.  Lenze have offered low wear friction material since 2004, but wear on other mechanical parts as the brake opens and closes can restrict life to about one million operations.  A new long-life design is now available where the sliding splines are coated in a special plastic developed in the space programme.  This reduces wear.  In addition guide pins and spring tappets take away the highly stressed areas that result from the cyclic armature plate movement.  The result is a huge increase in life with 10 million operations guaranteed and an expected life two times higher than that.  The latest Lenze inverters even have protected relays to switch the brake built in as a standard, removing the need for external components. 

Gearboxes are expected to operate reliably with higher motor input speeds.  This results in increased heating within the gearbox and an increase in internal pressure which in turn depends on the oil fill and the mounting position.  These factors can result in oil loss through seals and ultimately a reduction in service life.  To combat this Lenze have introduced gearbox ventilation as standard on all models of their G-motion range from 06.  The vent valves open if the pressure exceeds 0.2 bar which reduces oil foaming and oxidation.  The life of the gearbox seals is extended by between 25 and 100%.

With energy costs as a driving factor, plant operators will increasingly scrutinise Life Cycle Costs and Total Cost of Ownership for new machines.  Lenze can assist with the LCC calculation using standard forecasting models such as being developed by the German VDMA trade organisation.  Often the largest factor is the energy usage pointing towards the use of inverters to control speed to match demand, energy efficient motors and efficient gearing.  In one recent case, a five year forecast for drives at an end user plant was able to show that the optimum size selection is a function of the usage.  For single shift operation the LCC for a Lenze GST04 gearbox was marginally less than the next size up GST05.  However on three shift operation the GST05 repaid the higher purchase cost with a LCC which was 13% lower. This is a consequence of the longer service life of the bigger gearbox.

Correct sizing of a drive to the application conditions is critical for energy saving and TCO.  An undersized drive will fail and incur maintenance costs.  However an oversized drive, particularly if run below its optimum speed or at partial load, will exhibit low efficiency and therefore waste energy.  Lenze have adopted the concept of Rightsizing, sizing a drive from a broad range with models that are finely stepped, and using strong expertise from trained Engineers or specialist software tools.

The difference between price and cost can be vast – where price is defined as the purchase price off the machine and cost is the Total Cost of Ownership.  Plant operators need to look right through the life cycle up to disposal costs.  Lenze products offer good opportunities for recycling, for example the steel and iron content in gearboxes, and RoHS certification on the drives.  Elsewhere new developments from Lenze are making big increases in service life to meet the higher machine operating conditions. These improvements are available either as standard at no extra cost, or for only a small premium.

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