Mechanical Power Transmission

150 Pages·1971·28.55 MB·English

by  Peter C Bell BSc (eds.)

Mechanical
Power
Transmission
Mechanical
Engineering Series
ee
Palgrave Macmillan
Mechanical
Power
Transmission
Edited by Peter C Bell BSc
ee
Palgrave Macmillan
Published by
The Macmillan Press Limited
Technical and Industrial Publishing Unit
Managing Editor William F Waller
AMITPP AssiRefEng
General Manager Barry Gibbs
The Macmillan Press Limited
Brunei Road Basingstoke Hampshire UK
ISBN 978-1-349-01199-5 ISBN 978-1-349-01197-1 (eBook)
DOI 10.1007/978-1-349-01197-1
©The Macmillan Press Limited 1971
Softcover reprint of the hardcover 1st edition 1971 978-0-333-12546-5
SBN 333 125 460
Forevvord
The power developed by various types of engine may
be converted into widely differing forms of useful work,
up to a considerable distance from the prime mover.
The many types of mechanical power transmission used
in industry are discussed in this book.
The simplest form of transmission is a shaft joining, say,
an engine to a driven unit such as a compressor;
however, there are many types of shaft and appropriate
couplings required to meet industrial applications of
this basic drive (chapter 1 ); flexible shafts are discussed
in chapter 1 2.
To disengage the drive system, various clutches are
used, from simple manually-operated devices to fully
automatic mechanical, hydraulic, and pneumatic types
(chapter 2). Hydraulic and pneumatic systems are used
as single couplings or as complete drive complexes,
and are discussed in their various forms (chapters 6,
7, 8). Probably the most widely used industrial mech
anical transmissions involve one or more of the variants
that have been developed from the simple gearwheel
(chapter 3). The rope has long been used as a method
of transmitting mechanical effort over a distance; chain,
rope, and belt drives have developed extensively to
meet modern industrial needs, and these are discussed
in the book (chapters 9, 10, 11 ).
The types of transmission mentioned so far are used in
many fields of industry; there are certain areas for
which special designs of these transmissions have been
developed. These include: miniature gears (chapter 5);
instrument and control drives (chapter 14); variable
speed drives (chapters 4 and 13); oil-shear clutch and
brake systems (chapter 16).
Some mechanical drives incorporate brakes for their
correct function, and these braking devices are dis
cussed separately (chapter 15). The book is completed
by a glossary of terms and a guide to makers of trans
mission equipment.
Contents
Chapter Page Chapter Page
1 Shafts and Couplings 7 10 Rope Drives 93
D G Barnett MIProdE W R M Lindsay
Technical Director Technical Director
Standage Power Couplings Ltd Martin Black Wire Ropes (Northern) Ltd
2 Clutches 17
11 Belt Drives 103
A Gaunt
D H Ashworth MISE
Head of Clutch Design
Power Transmission Engineer
Crofts (Engineers) Ltd
Industrial Products Division
The Goodyear Tyre & Rubber Co (Great Britain) Ltd
3 Constant Ratio Gear Systems 25
K Lightowler CEng MIMechE
Product Application Manager 12 Flexible Shaft Drives 111
David Brown Gear Industries Ltd P J Wood
Technical Manager
4 Variable Speed Drives 37 The S S White Industrial Division of Pennwalt Ltd
D Gill
Technical Specialist
Crofts (Engineers) Ltd 13 Process Control by Variable Drive
J P Crum AssociTA I 115
5 Miniature Gears 45 Publicity Manager
Allspeeds Limited
R Nichols FCAe CEng AFRAeS AMCT
Technical Director
Reliance Gear Co Ltd 14 Instrument and Control Drives 121
6 Torque Converters and 55 P S Glover
Chief Designer
Hydraulic Couplings
Reliance Gear Co Ltd
A L Gatiss
Technical Consultant
Brockhouse Engineering Ltd 15 Transmission Brakes 131
S J Pinder
7 Hydrostatic Drives 69 Technical Director
J D Hamilton Elliston, Evans & Jackson Ltd
Chief Experimental Officer, Special Products Division
National Engineering Laboratory
16 Oil-shear Drive Systems 139
8 Pneumatic Drives 77 G M Sommer President
D J Bickley CEng MIMechE G M Sommer Co Inc
Director and P C Bell BSc MCIMM
The Globe Pneumatic Engineering Co Ltd
Guide to Equipment Makers 145
9 Chain Drives 83
Technical Staff
Renold Limited Glossary of Terms 153
Chapter 1
Shafts and
Couplings
D G Barnett M/ProdE
Standage Power Couplings Ltd
The need for a flexible coupling as a means of joining a
prime mover and driven machine is now universally
accepted. Its purpose is to provide a flexible element
between two machines, absorbing or minimising torsional
fluctuations and relieving bearings of loads imposed by
inaccurate alignment or subsequent movement.
Historically the earliest couplings were made to join lengths
of line-shafting and were normally of the solid flange or
split muff type. Difficulties in alignment prompted the
development of the Oldham coupling for parallel shaft
displacements, and engine connections were often through
a simple coupling consisting of cast hubs laced together
with leather belting. From this beginning many couplings
have evolved for various duties in a wide variety of designs
and materials. At present the designer is faced with an
almost bewildering assortment, and choice is often made
on price considerations or factors other than basic technical
merit. It is the aim of this chapter to review the main estab
lished designs of mechanical couplings together with their
applications and their relative merit. Reference to specific
couplings or trade names has been avoided. First, one
should consider some of the design factors affecting
couplings and clarify some of the terminology which is
used.
Line shafting, once a vital method of transmitting power in
industry, has now all but disappeared, and with it many
associated problems. The almost universal application of
the electric motor to individual drives has reduced machines
to simple close-coupled units. The remaining shafts can
vary between simple interconnecting devices to complex
shafts of specific torsional properties.
SHAFTING
Design and stress requirements of shafting is a subject well
documented in most technical reference books, I and it is
therefore not proposed to give extensive formulae here.
7
in gas turbine and traction drives, but it must be emphasised
that these are part of a transmission system produced as a
result of lengthy analysis, rather than a simple means of
joining two units.
A
COUPLINGS
The two most common coupling configurations are shown
in Fig. 2. In its simplest form the coupling consists of two
hubs joined by some flexible element. This type of coupling
is known as single engagement (Fig. 2A). The double
engagement equivalent (Fig. 2B) has two flexible elements,
B
and with hubs (as shown) it is a spacer coupling. With hubs
turned inwards it has the usual nominal shaft gap. Two
similar couplings mounted on the ends of an intermediate
shaft form a cardan shaft unit (Fig. 1A). It must be appreci
ated that the type of flexible element employed depends on
the coupling layout. That shown at Fig. 2A is normally
required to take angular and parallel misalignment, while
each element of the double or cardan shaft coupling will
Fig. 1. Types of drive shaft: (A) simple unsupported shaft only be required to accommodate angular misalignment.
(car dan shaft); (B) simple supported shaft; (C) complex shaft. Obviously the amount of parallel misalignment possible
in these designs is a function of the maximum permissible
angle and the length of the spacer or intermediate shaft.
Solid bright drawn mild steel (28-32 tons tensile; 1 ton=
1.016 t) is satisfactory for the majority of shafts of moderate Misalignment
lengths and speeds. A working stress of 560 kg/cm2 (8 000 The purpose of a coupling is to take up errors of shaft
lb/in2) is acceptable for this material. For more critical alignment and, depending on the particular design, to
applications ENS material (35--45 tons tensile) may be absorb or reduce torsional variations. The three modes of
specified, and shafting may be turned all over to ensure misalignment are shown in Fig. 3 somewhat exaggerated.
straightness and balance. For applications where weight It will rarely be found that a single mode of misalignment
and length are important considerations hollow shafting exists in any one drive and all three will be catered for to
has much to commend it. Holl.Qw material for shafting is varying extents according to the design of coupling
not easily obtained; commercial cold drawn tube appears employed.
a good proposition, but since its straightness is not good
and wall thickness non-uniform its use may be limited. A Fig. 2. Coupling configurations: (A) single engagement; (B)
better proposition is welded tube which has better straight double engagement coupling.
ness, and since it is formed from sheet its wall thickness is
uniform and therefore has superior balance.
In the design of shafting, one rarely has to work from
theoretical considerations only. A coupling or bearing size,
A
or some other fixed data usually forms a starting point.
Shafting normally falls into one of the categories in Fig. 1,
of which (A) is possibly most common, and more will be
said when discussing couplings for cardan shafts. In cases
(A) and (B) the aim is to provide a shaft of minimum diameter
compatible with the torque to be transmitted and the
length and speed. In example (C) due allowance must be
made for the weight of shaft-mounted pulleys and for the
reactions of the belt driving torques. Pulleys and couplings
are usually fitted with keys, which are discussed later. In
the design of shafts, care should be taken to avoid sharp
corners or changes of sections which would give rise to
stress concentrations. B
In certain applications a shaft may be called upon to perform
far more than mere transmission of power. A properly
designed shaft may greatly affect the running of a complete
system. Elaborate and costly contoured shafts are employed
8
a function of the transmitted torque and the coefficient of
friction of the particular element. Exceptions to this are
couplings indicated in Table 2.
Resilience
Almost all couplings are referred to as flexible or resilient,
and these two terms should not be confused. A flexible
coupling is one which will tolerate misalignment of the type
mentioned above, but it is not necessarily resilient. A
resilient coupling is torsionally soft as well as being flexible,
as already defined. The torsional resilience of a coupling
may vary from less than a degree to the order of 15°. The
B 1~----~
coupling stiffness is normally expressed in kgfm/rad (or
6 =¥~---t
lbfftjrad in British units). It is not always advisable to
select the softest possible coupling. Selection should be
made bearing in mind the relative inertias and torque
fluctuations of the coupled machines. A detuning coupling
is one in which the relationship of torque and twist is not
linear. The torsional characteristics of the drive are there
fore altered according to the torque transmitted.
Reversing drives
Virtually all couplings can transmit equal torque in either
direction, but special consideration should be given to
drives which are frequently reversing or in which torque
Fig. 3. Forms of misalignment: (A) angular; (B) parallel; reversals occur without a change in the direction of
(C) end.float. rotation.
Maintenance requirements
Ideally a coupling might be installed and forgotten, but no
How much misalignment? This is the question frequently coupling should be treated in this way, although it would
asked and most difficult to answer. The normal close appear that many are. At the very least, couplings which
coupled drive of medium to large size should be aligned are claimed to be maintenance-free should be given a
with the utmost care; the better the alignment, the longer periodic examination to ensure the condition of the drive
the coupling life. The fact that a coupling can take mis element. All metal couplings usually require regular lubri
alignment should never be made an excuse for poor cation, and it may be necessary to remove covers to check
workmanship. the condition of elements. Rubber couplings generally
require little attention, but they should be periodically
A reasonable figure for a single element coupling would be carefully checked since rapid deterioration may occur
0.25-0.40 mm (0.010-0.015 in) parallel misalignment or -!0 without warning.
angular. Where both types of misalignment are present in
combination, individual amounts would be less. Some rub Overload protection. It is sometimes convenient to combine
ber couplings can greatly exceed this, say up to 0.8 mm a normal flexible coupling with an overload protection
(0.030 in) and 5°. These figures are however subject to device. Such devices are usually of shear pin design or a
individual drive conditions of which the main factor is form of slipping clutch. Prospective users should consult
speed. For drives where a certain definite misalignment an appropriate manufacturer.
must be present, say the drive to a flexibly mounted machine,
it is common to use a spacer type coupling or cardan shaft.
DESIGN FACTORS
With this, useful amounts of misalignment can be obtained
with normal couplings, subject to the length of the inter The majority of couplings fall into the simple types (Fig. 2)
mediate portion. consisting of a driving and driven hub with a flexible centre
member of some suitable design. When considering
End float couplings for a particular drive the following factors should
Most couplings can take end float to varying degrees, some be considered.
lend themselves to telescopic movement; eg gear type. The
main consideration here is the load needed to slide the Drive
coupling under static or dynamic loaded conditions. Most (1) Torque or power to be transmitted.
couplings cater for end movement by sliding the element, (2) Torque variations and frequency.
which at very best can approximate to a spline but is (3) Whether reversing.
probably very much worse than this. Therefore the load is (4) Speed variations.
9
(5) Starting torques. and all-steel construction is almost certain to be necessary;
(6) Shaft sizes and types. high carbon or alloy steel may have to be used. All projec
(7) Key size and type. tions should be avoided to minimise air resistance and
(8) Drive conditions, shock load, etc. whistle. Since all-metal couplings are appropriate to these
(9) Misalignment. conditions, special attention may have to be given to
(10) End float. lubrication.
(ll) Temperature or abnormal ambient conditions.
(12) Type of driving and driven machines. Keys, keyways, and mounting
(13) Position and inertia of any flywheel. Small couplings for fractional-power (less than 750 W or
(14) Whether flange or flywheel mounting required. 1 hp) drives may be a sliding fit on their respective shafts,
secured by grub screws or pins. A key should be used on
Coupling shafts of approximately 13 mm (0.5 in) or larger; the key
(1) Quantity. may be parallel or tapered according to personal preference.
(2) Type. Parallel keys are often used on sizes up to 76 mm (3 in) and
(3) Compatibility with above drive conditions. tapered keys on larger sizes; there is however no hard and
(4) Maximum bore size. fast rule. The keys used should be in accordance with the
(5) Initial cost. relevant standard.2 Currently, coupling manufacturers are
(6) Relative cost of replacement drive elements. asked to cut keys to the British 1929 standard, the 1958
(7) Maintenance requirements. standard, and for non-standard and metric sizes. Multiple
(8) Ease of installation. keys are sometimes used in larger size shafts and drives
(9) Interchangeability of parts. subjected to particularly heavy loads.
(10) Adaptability.
(11) Weight and inertia. The coupling must be a good fit on its shaft. Exact amounts
(12) Maximum speed and balance. of clearance or interference are matters of personal pre
ference, but small shafts (say up to 33 mm; 1.5 in) are often
These conditions are largely self explanatory but some made a sliding fit while larger shafts are usually made an
clarification may be needed. The coupling should be easily interference fit. Coupling manufacturers, if left to their
separable into two halves without any axial displacement own discretion, usually provide hubs bored to a light
of either driving or driven machine. This allows easy interference fit. It is therefore important to specify particular
removal of, say, a motor armature or gear shaft without requirements.
disturbing foundations. Many designs of couplings parti
cularly in smaller sizes do not fulfil this condition and this Keyless drives
may or may not be important on the particular application. Gaining popularity is the tapered bush method of fitting
Designs should be adaptable for different drive conditions. coupling hubs. These eliminate the need for keys, although
It may be possible to vary the torsional resistance and to a key may sometimes be fitted. A typical bush is shown in
limit the misalignment capacity to angular only for use in Fig. 4, which shows that the coupling is bored tapered to
spacer or cardan shaft drives. suit the split bush, which can be forced in with two screws.
This action compresses the bush so that a satisfactory drive
Coupling materials can be obtained; the bush can be released in a similar man
Coupling hubs can be made in most engineering materials, ner. The main advantage of this system is that keys and
commonly: steel, cast iron, sintered iron, zinc alloy, keyways can be eliminated, shaft sizes are not so critical,
aluminium alloy, and certain plastics depending on the size and bushes with different size bores can be stocked to
and duty involved. Small couplings can be economically
diecast, while larger units are produced from fully machined
steel forgings. Fig. 4. Typical taper bush.
Speed
Maximum speeds of couplings are normally quoted by
manufacturers. Speed may be limited by the centrifugal
loading on the elements or the coupling hub material. In
the absence of any other limiting factors it is usual for the
linear speed of the largest diameter of a cast iron coupling
not to exceed 30 m/s (100 ft/s). A steel coupling may be
run at double this speed. For high speed drives, special
attention must obviously be paid to coupling balance. Up
to the above limits the normal coupling machined all over
is usually satisfactory, but where higher operating speeds
are required special factors must be considered. The overall
diameter should be as small as possible, compatible with
torque capacity. Dynamic balancing should be employed,
10
DRIVEN MACHINE FACTORS Grinding. crushing, screening Pumping and compressing
Steelwork plant plant plant
Forging machine (belt or chain Rotary screen 1.3 Blower 1.25
driven) 2.0 Grinding mill 1.5 Fan 1.25
Wire mill 2.0 Ball mill 2.0 Pump (centrifugal) 1.25
Shearing machine 3.0 Pulveriser (coal) 2.0 Exhauster 1.2
Bar straightening machine 3.5 Cement mill 2.25 Rotary compressor 1.5
Tube mill (drawbench) 3.5 Disintegrator 2.25 Mine fan (ventilating) 2.5
Forging machine (direct driven) 4.0 Rubber mixer 3.0 Pump (ram) 3.0
Cane knives 3.0 Compressor (quadruplex
Steelworks cranes (Reversing Joggling machine 3.5 radial type 2.0
and with brake drums) Ore crusher (large gear Compressor (reciprocating) 3.0
Main hoist 3.5 reduction) 4.0
Auxiliary hoist 3.5 Roller cane mill 4.0 Miscellaneous
Slewing 3.5 DC generator 1.25
Long travel 2.5 Conveying and hauling Alternator 1.3
Main cross traverse 2.0 machinery Welding generator 2.0
Auxiliary cross traverse 2.0 Conveyor (belt) 1.25 Textile machinery 1.25
Automatic boiler stoker 1.5 Propeller (marine) 2.5
Other cranes Winch and capstan 2.0 Paddlewheel (marine) 2.7
Main and auxiliary hoist. reversing Winder and haulage 2.5 Electric or hydraulic steering
and with brake drum 3.0 Suction elevator (grain) 2.75 gear 2.5
Long travel, reversing and with Passenger and goods lifts 3.5 Bar reeling machinery 2.5
brake drum 2.0
Main and auxiliary cross traverse 1.75 Paper-making machinery PRIME MOVER FACTORS
Rotary screen 1.3 Turbines and electric motors 0
Wood and metal-working Brush doctor 1.5 Steam and petrol (gasoline)
machinery Drying cylinder 1.5 engines 1.3
Light wood-working machinery 1:25 Couch roll 2.0 Oil (paraffin) engines 1.5
Machine tools. (Excluding Press roll 2.0 Gas engines: 1 cylinder 2.7
planing machines) 1.25 Calender rolls 2.7 2 and 4 cyl 2.5
Wood-planing machinery 1.5 Pulper 3.5 Diesel engines: 6. 7, 8 cyl 2.0
Sawing machinery 2.0 4 cyl 2.25
Heavy wood-working machinery 2.0 1. 2, 3, 5 cyl 3.0
Planing machines. reversing
(metal) 2.5
Table 1. Typical drive factors used as a guide to the selection of suitable couplings.
simplify mounting. Furthermore, time taken in key fitting, COUPLING RATING AND SELECTION
often by skilled personnel, is eliminated. Their disadvantages
are the higher initial cost of the bush, plus taper boring, The capacity of a coupling is quoted in manufacturers
drilling and tapping of hubs. They also tend to reduce the tables as its torque capacity usually as a ratio of hp (kW)
maximum parallel bore which can be tolerated in a boss of per rev/min or hp (kW) per 100 rev/min; figures of torque
given diameter. Taper bushes are available in a range of (kgfm, lbfin, lbfft) are often quoted in addition. This
sizes covering bores of 9.5-127 mm (i-5 in). capacity is the resilient capacity of the coupling and is
normally independent of speed. The ultimate or yield
Larger shafts and bosses can be provided with keyless drives capacity, not usually quoted, would be many times greater.
by shrink fitting or preferably by the hydraulic fit method, a
which consists of expanding a hub by hydraulic pressure The general formulae for power and torque are:
and fitting by pressing on at loads which are reduced by
interposing an oil film. The design requirements of this T orque= hp X 3. 3 000 lbf ft kW X 6105 kgf m
rev/ mm x 2n rev /min x 2n ·
method are quite critical, but for heavy and difficult
applications the system is excellent. These can be simplified to:
Torque (coupling capacity)= hp/(revjmin) x 5252 lbfft
Special shafts
=kW/(rev/min) x 972 kgfm.
So far, only plain round shafts have been considered.
Certain drives are through tapered shaft ends with key and Coupling selection
clamping nuts; typical of these are marine propellers and A standard selection procedure which is applicable to all
standard mill motors. Splines and serrations are used where types of couplings is given by the formula:
the design calls for sliding movement or special location. p
Both methods should however be considered for quantity T=ji.F1 +F2)
production since they are difficult to produce economically
in small numbers unless special tooling is available. where T=torque; P=normal drive power; N=normal or
11