Page 55 - Automotive Engineering

CH AP TER 2 .1 Measurement of torque, power, speed and fuel consumption

2.1a.10.7 Alignment of engine and a failing shaft from whirling; these should not be so close
dynamometer to the shaft as to be touched by it during normal starting
or running otherwise they will be the cause of failure
This is a fairly complex and quite important matter. For rather than a prevention of damage.
a full treatment and description of alignment techniques,
see Ref. 4. Differential thermal growth and the move-
ment of the engine on its flexible mountings when on 2.1a.11 Engine to dynamometer
load should be taken into account and if possible the coupling: summary of design
mean position should be determined. The laser-based procedure
alignment systems now available greatly reduce the effort
and skill required to achieve satisfactory levels of accu- 1. Establish speed range and torque characteristic of
racy. In particular, they are able to bridge large gaps be- engine to be tested. Is it proposed to run the engine
tween flanges without any compensation for droop and on load throughout this range?
deflection of arms carrying dial indicators, a considerable
problem with conventional alignment methods. 2. Make a preliminary selection of a suitable drive shaft.
There are essentially three types of shaft having dif- Check that maximum permitted speed is adequate.
ferent alignment requirements to be considered: Check drive shaft stresses and specify material. Look
into possible stress raisers.
1. Rubber bush and flexible disc couplings should be
aligned as accurately as possible as any misalignment 3. Check manufacturer’s recommendations regarding
encourages heating of the elements and fatigue load factor and other limitations.
damage. 4. Establish rotational inertias of engine and dyna-
2. Gear type couplings require a minimum misalign- mometer and stiffness of proposed shaft and cou-

ment of about 0.04 to encourage the maintenance of pling assembly. Make a preliminary calculation of
an adequate lubricant film between the teeth. torsional critical speed from eq. (1). (In the case of
large multicylinder engines consider making a com-
3. Most manufacturers of universal joint propeller
shafts recommend a small degree of misalignment to plete analysis of torsional behaviour.)
prevent brinelling of the universal joint needle 5. Modify specification of shaft components as neces-
rollers. Note that it is essential, in order to avoid in- sary to position torsional critical speeds suitably. If
duced torsional oscillations, that the two yokes of the necessary, consider use of viscous torsional dampers.
intermediate shaft joints should lie in the same plane. 6. Calculate vibratory torques at critical speeds and

Distance between end flanges can be critical, as incorrect check against capacity of shaft components. If nec-
positioning can lead to the imposition of axial loads on essary specify ‘no go’ areas for speed and load.
bearings of engine or dynamometer. 7. Check whirling speeds.
8. Specify alignment requirements.
2.1a.10.8 Guarding of coupling shafts
9. Design shaft guards.
Not only is the failure of a high speed coupling shaft po-
tentially dangerous, as very large amounts of energy can be
released, but it is also quite common. The ideal shaft- 2.1a.12 Flywheels
guard will contain the debris resulting from a failure of
any part of the drive line and prevent accidental or No treatment of the engine/dynamometer drive line
casual contact with rotating parts, while being sufficiently would be complete without mention of flywheels which
light and adaptable not to interfere with engine rigging may form a discrete part of the shaft system. A flywheel
and alignment. A guard system that is very inconvenient to is a device that stores kinetic energy. The energy stored
use will eventually be misused or neglected. may be precisely calculated and is instantly available. The
A really substantial guard, preferably a steel tube not storage capacity is directly related to the mass moment of
less than 6 mm thick, split and hinged in the horizontal inertia which is calculated by:
plane for shaft access, is an essential precaution. The 2
hinged ‘lid’ should be interlocked with the emergency I ¼ k M R
stop circuit to prevent engine cranking or running while it where:
is open. Many designs include shaft restraint devices
2
loosely fitting around the tubular portion, made of wood I ¼ moment of inertia (kg m )
or a non-metallic composite and intended to prevent k ¼ inertia constant (dependent upon shape)

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