Pneumatic or Electric Doors for Double-deckers?
Page 44
Page 45
If you've noticed an error in this article please click here to report it so we can fix it.
OF the two systems available for actuating the doors of the rear platform of double-deckers, the electrical system seems to be the more popular. Pneumatic equipment, however, is common for front and central doors of double-deckers, and on singledeckers.
With the electrical system, as offered by C.A.V., Ltd., London, W.3, a reversible motor with a worm reduction drive is mounted centrally above the doors. A double lever above the gear shaft works a door shaft at each end. Rotation of the motor one way turns the double lever through 90 degrees and this opens the doors by rotating the ,hafts. Reversal of the motor closes the doors.
Between the reduction gear and the double lever is an overload slipping clutch which permits opening of the doors by hand if power fails. If there is an obstruction in the doorway or runners, the clutch will slip and prevent harm to the mechanism or a passenger.
• Three-second Operation
Each button. for " open " and close." works a relay in a switchbox near the doors. Fuses are provided or there may be a hi-metal strip which breaks the circuit when overheated. The driver has a reset button to press when the strip has cooled. Opening or closing of the doors takes three seconds, and uses 0.01 amp.hr. of current_ The load is 20 amp. at 12v., or 8 amp. at 24v.
The motor must be stopped at the moment when the door reaches the end of its travel, or it will slip and the doors will slam. The 'linkage Must he adjusted so that both doors are fully open and closed at the same moment_ Movement is limited by two stop switches worked, one in each direction, by an adjustable striker al0 At Reading, the transport department found that door hinges became worn after a time, and the resulting play let one striker work its switch earlier or later than it should. On new vehicles the switches are worked by adjustable cams on the reduction-gear shaft. As the gear rotates, the cams work leafspring limit switches on the casing.
Reading may be said to be the proving ground for power doors on double
deckers, '1 eething troubles have been encountered, but have been solved by the makers. Wear on the keyways of the main motor spindle, because of frequent reversal of rotation, was dis-. covered. Formerly, the spindle could not be renewed without also obtaining a new worm wheel as these two components had to be trued for accurate centring. The latest design, however, uses a spline instead of a keyway.
• The department, which has used rear power doors for a much longer time than most other operators, has found little need for maintenance. Other operators' experience is similar. Life of the equipment between overhauls is equal to that of the rest of the bus, and no intermediate attention is needed. Trolleybuses have a device by means of which they cannot be driven off if anyone is trapped in the doors.
Oil-engined buses with vacuumservo brakes can use pneumatic equipment to work the doors. This is satisfactory with a single-decker, but on a heavier double-decker all the vacuum reserve is sometimes needed for road hrakine. If the bus has pneumatic brakes, air is available for actuating the doors. Makers of vacuum aud compressed-air equipment arc G. D. Peters and Co., Ltd., Slough, Bucks. Deans and Son (Yorkshire), Ltd,,, Beverley, manufacture tie rods and levers.
Pneumatic control is suitable for front doors worked by the driver or for rear doors worked by the conductor. If control of rear doors is to be in the hands of the driver, the piping run is long, which leads to delay in action, so an eteetro-pneumatic valve may be used.
When its cod is energized, two relay contacts, built into the valve frame, are closed and a positive feed is maintained to the coil after the button is released. The air then holds the doors closed until the "open " button is pressed.
The door unit works. at 40 p.s.i., with a differential cushioned action. With a single folding door, the unit is linked to the door shaft by an adjustable connecting rod, and with a double door the two shafts are linked by a chain drive with an adjustable terminal eyebolt.
Adjustment in Lining Up Doors are hung and pivoted on a tubular shaft by special straps with locking nuts which enable adjustments to be made during lining up. • The door shafts are carried in combined journal and thrust hall bearings, which accommodate-the weaving action of the body. Correct alignment is important.
Air and electric doors are equally satisfactory, and there is scope for both. Air doors may sometimes be more hesitant in operation, and may tend to freeze in cold weather, but they have fewer components and are less expensive to install.
Weight of power doors and their mechanism, with alterations to the body, is at least 11 cwt. Most makes of body can support this without reinforcement. and it is not necessary to stiffen the rear springs on the chassis, but the rate of fuel consumption increases fractionally.
Reading Transport Department's installation provides for an interlock which preveptithe doors from opening while the bus is moving under power, and prevents the bus from starting until the doors have closed. When the doors are closed, two green lights appear in the cab. Both opening and closing of doors are done by the driver, and the conductor has only to ring the bell. There is a button under the stairs, hidden from the view of passengers, by means of which the conductor can open the doors if the driver forgets.
A traction switch, or door-interlock cancelling switch, in the cab can be used to short-circuit the doors. This allows the vehicle to move short distances with them open, if need be, and lets the conductor leave a trolleybus to work the overhead frogs.
Warning Lights
When this switch is in use, two white lights appear in the cab, where they are also visible from outside by an inspector. If doors become defective, they can be cut out, and the bus run with them open,
Walsall Corporation bought 10 Leyland double-deckers with Roe highbridge type bodies in 1953 for use on routes to Rugeley and Stafford. The saloon-floo,level was, carried through to include the platform, and electrically operated double jack-knife doors were fitted.
The same type of door was introduced for urban service, at the end of 1954, on 15 Sunbeam trolleybuses with Willowbrook 70-seat bodies. These vehicles have an opening of 3-ft. 7-in. width.
Most power doors are of two-leaf, or double jack-knife type. in which two leaves fold against the forward partition and two against the rear of the bus. All hand-worked designs have two leaves only, of double the width, and both fold against the partition.
Two-leaf Doors
Exceptions are three bodies built in 1949 by Wilks and Meade, Ltd., on Daimler chassis for Premier Travel, Ltd.. Cambridge. These have two-leaf doors which fold against the rear, and to make room for them the foot of the stairs is farther to the off side than normal. They were originally vacuum operated, but are now worked by hand.
The glider type of door, which is pivoted at its centre and popular for single-deckers, cannot be used on a double-decker as it is unsuitable for openings wider than 2 ft. 6 in.
The Tilling type of two-leaf door hangs on two rollers at the top. When not in use it can he fixed open by a spring clip near the rear hinge of the forward leaf, which engages with a hook on the partition.
With most types of door, the usedticket box has to be moved to avoid being obscured. On fnost four-leaf
designs it can remain in the usual place if it is moved nearer the centre-line,of the bus, and is somewhat small. If a larger box is needed, it must be on the inside of the emergency door.
With two-leaf doors the box is usually on the inside of the rear leaf, moving into the usual location when the doors are open. On vehicles of the Birmingham and Midland Motor Omnibus Co., Ltd., and Southdown Motor Services, Ltd., windows arc fitted below the waistline of doors as well as above.
At least three operators in 1953-55 fitted doors to open-platform buses. Each conversion involved 1951-52 Leyland Titans with Leyland metal-framed bodies. Southdown modified 24 in their own works at Portslade at a cost of between £210 and £260 per bus. Maidstone and District Motor Services, Ltd., sent 28 to Weymanns, Ltd., Addlestone, for conversion, and Stratford-upon-Avon Blue Motors, Ltd., had three converted by Willowbrook, Ltd., Loughborough.
At the end of 1945, London Transport fitted air-operated double jack-knife doors to a 1936 A.E.C. six-wheeled trolleybus with B.R.C.W. body. Doors were also fitted to an RT motorbus with 1939 Chiswick-built body. Both ran for a few months in the Kingston area, but were soon withdrawn.
The motorbus had a three-piece sliding door and resembled the Southlanco equipment described in Therommercial Motor, dated June 15, except that the Southlanco is electrically actuated and has two leaves which move simultaneously, one faster than the other.