Page 64 - Electric Drives and Electromechanical Systems

Chapter 2 Analysing a drive system 57

mechanical aspects this must also include details of the operating environment; for if
these details are not considered at an early stage, the system which is selected may not
be suitable for the application.

2.7.1 Mechanical compatibility
The mechanical requirements of the motor must be identified at an early stage in the
sizing and selection procedure. Items that are frequently overlooked include any
restrictions in dimensions and orientation resulting from the mechanical design. If
such restrictions can be identified at an early stage unsatisfactory performance of the
equipment after it is installed may be prevented. For example, when the motor is
mounted in the vertical position, special shimming or bearing preloads may be
required. Apart from ensuring that all the mechanical parts fit together, the problems of
assembly and maintenance should be considered at this stage; there is no more frus-
trating (or costly) problem than a junction box that cannot be reached or having to
dismantle half a machine to replace a motor or position encoder.
The correct identification and determination of the load and any externally applied
forces often is the most critical aspect of sizing a motor-drive combination. The worst-
case masses, forces, and speeds need to be accurately determined if the motor drive is to
be correctly sized. Even if the exact mass of a component is unknown, it can be deter-
mined from the component’s volume and density.
In an application where the linear-motion axis is horizontal, these values can nor-
mally be determined without significant problems. However, when the axis of motion is
vertical, the total weight of the load, if it is not counterbalanced, will appear as a
constant load on the motor, which must be taken into account. When the load is
counterbalanced, the design must be carefully analysed. Two possible counter-
balancing schemes can be used, either using a second mass, or a pneumatic system. If
the former strategy is used, the effective load inertia must include the counterbalance
when the load’s operating requirement is calculated. If a pneumatic system is used,
Fig. 2.12B, to support the load, the percentage of the load which is supported must be
determined, and the motor must again handle the unsupported load as a constant-
thrust loading. Systems of this type should be carefully designed to ensure there is
no possibility of damage because of a failure of the system. This may require the fitting
of brakes, overspeed detectors, and, in the case of the hydraulic or pneumatic systems,
pressure switches.
The area that most often prevents successful motor-drive sizing is poor estimation
of the frictional forces. Modern antifriction devices such as air bearings, hydrostatic
ways, linear-roller or ball-thrust bearings have done much to minimise this problem, as
discussed in Section 3.4. In most cases the frictional forces can be safely estimated for
horizontal motion using Eq. (2.26). Again, in a vertical application, the forces on the
slideways need to be carefully resolved. If the drive is being used as a retrofit on an
older design of machine tool which incorporates dovetail slides or similar bearings, the