| Make |
Trailer, Cable Drum (3 Wheel Type) |
| Model |
R.C. Gibbons |
| Type |
Large cable Carrier (3 wheel) |
| Body Builder |
|
| Use |
Cable drum transportation |
| Registration Number |
|
| Fleet Number |
|
| Date of picture |
1951 (bottom picture 1958) |
TRAILER, CABLE DRUM (3-WHEEL TYPE)
Reference
P.O.E.E.J. Vol. 48, Page 202.
Application
Provides a means of transport for all standard P.O. cable drums up to 7ft. 6in.
diameter, 3ft. 8in. wide and 4tons in weight.
General description
The trailer consists of an open framework mounted on two high-pressure
pneumatic-tyred road wheels, with a retractable castor wheel at the forward end
which is used for balance and manoeuvring when detached from the towing vehicle.
Two screw jacks and a centre spindle provide a means of mounting the cable drum.
The ends of the open framework are closed by means of a detachable crossbar
which relieves any fatigue that may occur in the welded assembly. Two rear
steady legs are also provided for additional stability during cabling
operations. The cable drum centre spindle is also provided with adjustable end
stops to prevent sideways movement of the drum. The trailer is equipped with
over-run brakes and vacuum brakes.
Dimensions
Length 11ft. 6in. Width 7ft. 6ins. Weight 19cwt.
Transport
May be towed by 1ton, 30cwt., or 2 ton G.U. vehicles but when the load exceeds
25cwt. a 4ton vehicle equipped with vacuum brakes extension must be used.
Capital cost
£400.
Remarks
For towing, caster wheel must be raised to full extent, swung forward and locked
to give maximum road clearance.
An article from
The Post Office
Electrical Engineers' Journal
Volume 48, Part 4 - January 1956
A New Cable-Drum Trailer
This article describes a new cable-drum trailer,
incorporating several novel features, which can be used for transporting cable
drums weighing up to 4 tons.
INTRODUCTION
To the layman the handling of cable drums probably appears a simple matter,
but when considered on a large scale a number of important engineering, economic
and legal requirements must be satisfied. It is essential that any
cable-drum-carrying device shall be as light in weight as possible, consistent
with the ability to carry the heaviest drum which it will be required to
transport, so 'that running costs are not increased by repeatedly moving large
masses of ironwork during the life of the device. The weight of a trailer also
determines certain legal requirements, since the Road Traffic Acts permit
trailers constructed to carry separable loads and having an unladen weight of
under one ton to have their brakes operated on the over-run principle. Hence, a
cable drum trailer weighing less than one ton may be towed by any vehicle
capable of· towing the combined load of the trailer and full cable drum,
irrespective of the vehicle braking system. Large vehicles used to convey heavy
·drums to a works site can be released for other duties and the empty drums and
lightweight trailer may be returned to depot using the gang vehicle. The new
cable-drum trailer described in this article has the advantage of light weight
and also incorporates a number of novel features.
DESCRIPTION OF NEW TRAILER
A pair of heavy-duty road wheels supports the main chassis framework, which
consists of an open-ended assembly that may be wheeled around a drum or into
which a drum may be rolled-the alternative methods catering for different
conditions such as unmade ground, restricted site conditions, full or empty
drums, etc. When in position within the framework the drum is raised on a
special spindle by means of a pair of lever-operated screw jacks mounted on the
framework. A tie-bar is then clamped across the open end of the framework. The
laden trailer is then coupled to the towing vehicle and the small front castor
wheel raised.
The simple operations involved reduce the time and manpower
taken to prepare a cable drum for road transportation to a small fraction of
that required for other methods, and all the operations are performed without
risk of mishap or injury to the personnel concerned. On arrival at the working
site, the laden trailer may be suitably placed and the cable fed directly off
the drum without further preparatory operations-other than removing the battens
from the drum.
Main Framework
To obtain the required simplicity in use the main framework has been designed in
the general shape of a wishbone. By so positioning the load that its weight is
centred and supported as directly as possible over the main road wheels, the
stresses in the framework due to the static load are greatly confined and are so
simplified that the predominating requirement in the design of the framework
becomes one of catering for the dynamic stresses, i.e., the stresses that arise
due to the momentum of the load when travelling, starting and stopping rapidly,
crossing uneven ground, etc.
The tie-bar clamped across the open end of the frame work
greatly limits the independent flexing of the limbs and consequently the fatigue
that could arise on the welded joints of the framework if the limbs were free.
To provide a firm attachment capable of meeting the stress requirements while at
the same time keeping the tie-bar light enough for easy handling, use is made of
a 4in. diameter hardened aluminium-alloy tube containing at each end a left hand
and right hand threaded screw operating a pair of jaws. The jaws engage on
dovetail projections provided at the end of the main limbs of the framework. The
towing yoke is less highly stressed and is therefore fabricated from
rolled-steel sections smaller in size than the main limbs, although large enough
to cater for the lateral stresses due to yawing.
Suspension
In an earlier design the two main road wheels were supported on stub axles but
this was quickly proved to be unsatisfactory under overload test conditions due
to the torsion applied to the main members. To eliminate this torsion the road
wheels have been enclosed in a rectangular sub-frame that is hinged to the main
framework at the front and supported on substantial coil springs (compression)
at the rear. The wheel axle is of the push-out type and is equally supported at
each end by the sub-frames. This unorthodox method of suspension has largely
eliminated the flexing of the main framework and has proved very satisfactory
under a wide range of field conditions.
Even when carrying a test load of 4.5tons and travelling over
specially selected rough, undulating ground at the highest speed the vehicle
could be driven on such terrain, the load did not bounce but "floated" gently.
Jacking Features
To enable the cable drums to be raised to their travelling position when within
the framework, a screw jack is provided on each of the main framework limbs.
These jacks are operated from a position outside the main road wheels by means
of a ratchet lever. To enable the operating levers to be at a uniform height
irrespective of the great (for screw jacks) travel of the lifting hook, the
motion is transmitted to the top of the jacks and applied to the main lifting
screws by spur gearing. The effort required to raise a drum of maximum weight is
of the order of 50lb. Where the drum has been raised sufficiently the operating
levers are latched in a position parallel to the main limbs of the framework.
The main lifting hooks have phosphor-bronze bushes to permit
easy running on the steel lifting screws, and the actual hook portion contains a
pin to secure the cable-drum spindle when travelling unladen over rough ground.
Adjustable cheeks are provided on the cable-drum spindle to enable the drum to
be centralised on the spindle or to enable a wide drum to be so positioned that
the tail end of the cable, which usually projects through the flange of the
drum, will not be damaged when the drum is rotated to pay off cable.
For easy wheel-removal for tyre maintenance, etc., a simple
one-piece prop is engaged on either of the cable-drum lifting hooks so that, by
lowering the lifting hook, the trailer wheel can be raised, allowing the wheel
axle to be drawn out and the wheel removed. The prop is stowed on the trailer
inside one of the channel sections forming the main framework.
Braking Arrangements
Although legally it is necessary only to provide an over-run braking mechanism
on trailers weighing less than 20cwt., it was considered wise to provide, in
addition, vacuum-operated brakes for use when heavy drums, i.e., 3tons or more;
are to be carried. The larger vehicles capable of towing a gross trailed load of
4 tons or more are being equipped with vacuum-operated brakes, and by a simple
extension of the pipe line on the vehicle it is possible to extend the system
over a flexible connection to the trailer. The trailer braking cylinders then
operate simultaneously with the vehicle brakes. Road tests showed that the
over-run brakes alone were slightly more effective than the vacuum brakes alone
(4·9 per cent. difference) whereas the combined effect of both brakes gave a
maximum retardation and tended to skid the trailer wheels.
Other Facilities
With the concentration of the load close to the road wheel centres, the laden
trailer is approaching a condition of balance. Actually, a downward load on the
tow bar is required and is provided in order to obtain good vehicle road-towing
conditions. This condition, of approaching a balance, does however have a minor
disadvantage; namely, that the centre of gravity of the drum may lie in a plane
near to the plane of support given by the road wheels. This condition can arise
only when the trailer is parked on a steep hill and a heavy drum is raised very
high on the jacks. To prevent any tendency for the trailer to rock under this
condition, and also to steady the trailer when cable is paid off the drum and
men clamber about the trailer to rotate the drum, a pair of adjustable steady
legs
is provided on the rear ends of the main limbs.
To meet legal requirements and prevent unauthorised persons
releasing the brakes when the trailer is temporarily parked and unattended, a
means of padlocking the parking brake in the "on" position is provided. The
front castor-wheel has been the subject of many changes during development.
Originally the castor-wheel was raised only by a screw action but field
conditions proved this to be very vulnerable to damage. The length of the
exposed screw necessary to cater for various heights of vehicle towing bars
prevented adequate support being provided and, although the castor wheel
appeared at first sight to have been raised sufficiently to clear the ground by
an adequate amount, at intersecting road cambers, ruts, pot holes, hump-backed
bridges, etc., the clearance proved inadequate. The present method involves
raising the castor wheel a few inches by means of a screw action, and then
unlatching the complete assembly and swinging it bodily forwards and upwards,
securing it automatically in a position largely within the towing yoke and with
a very considerable ground clearance.
The detachable handle used for retracting the castor wheel,
and the tools for day-to-day maintenance, are accommodated in a tool-box formed·
by boxing-in a portion of the channel section limbs forming the main framework.
CONCLUSION
The new trailer provides a compact lightweight means of transporting cable drums
up to 7ft. 6in. in diameter and 3ft. 8in. wide and up to 4 tons in weight, with
a minimum of ineffective time and labour in loading and unloading and a maximum
of flexibility of use with a wide variety of towing vehicles. Cable drums
exceeding these limits are sometimes encountered, but are rather exceptional and
are outside the range of standard P.O. drums for which the trailer has been
designed. In conclusion, it is desired to express appreciation of the
co-operation given in the development and production of these unorthodox
trailers by the staff of Messrs. R. C. Gibbins & Co., Ltd
Later type - Picture dated 1958
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