Electrokinetica The Electro-mechanical Museum

1967 Lansing Bagnall Electric reach-truck, a.k.a ‘Nellie’

Reach truck

Nellie is a bona-fide working piece of plant, responsible for moving our collection around the warehouse and assisting with all sorts of lifting jobs. As a result, she is not always the cleanest exhibit on the premises, although her running gear is carefully looked after these days. Almost every exhibit we own under 3300 lbs weight has been loaded or unloaded with Nellie at least once, so she is kept rather busy.

Vital Statistics:

Why a reach-truck?

When Lansing Bagnall introduced the FRER3 series of trucks in 1961 under the name ‘Spacemaker 3300’, they boasted a novel drive system with the motor and gearbox mounted inside one of the wheels, allowing extremely tight turns to be made. This was an important benefit because the aim of a reach-truck is to provide ultimate manoeuverability for working in tightly packed warehouses. It is so called because it can ‘reach’ out with its mast and forks to pick up a load, then retract it within its wheelbase to minimise the overall laden vehicle length. This contrasts with a standard counterbalance truck on which the load always overhangs the front wheels. A reach truck driver can arrive alongside a pallet rack and pivot about one of the load wheels to face it, i.e. the aisle width required is simply the length of the truck. A load can then be collected by reaching out, picking the pallet and reaching back in so that it is withdrawn from the racking, allowing the truck to turn back to face along the aisle again and depart. By convention, ‘forwards’ on a reach-truck is away from the forks. The driver sits sideways so he can see ‘ahead’ by looking left and ‘behind’ i.e. towards the load-handling operations by looking right. To the newcomer, reach truck steering appears to work in reverse when approaching a load, opposite to that of a counterbalance truck.

See the reach truck in action at the end of a switchgear sortout day at the stores. Watch video: Nellie collects a pallet of switchgear

Electric drivetrain

Drivetrain location

Drivetrain location

Hub motor

Hub motor

Most vehicles that drive and steer with the same wheels (e.g. front-wheel-drive cars) have only a limited steering angle, over which the drive can be transmitted to the wheels via driveshafts with constant-velocity joints. To increase the steering angle to 180 degrees, Lansing Bagnall introduced a self-contained powered hub drive in the FRER3 reach trucks, with the electric motor and gearbox built into the wheel itself. As a result an extremely compact motor is needed despite its 3 hp output at 1750 rpm. It is a silent-running shunt-wound four-pole machine built for Lansing Bagnall by Mawdsleys. The motor works in all four ‘quadrants’ i.e. forward accelerate, forward brake, reverse accelerate & reverse brake, under which conditions a shunt-wound motor is much easier to control than a series one. Shunt winding is therefore used, however, one of the desirable characteristics of a series motor (that of increasing torque capability with load) is simulated by the controller as explained below. The motor frame is integral with the hub assembly, while the shaft carries the sun gear of a planetary reduction gearbox, of which the planet carrier is connected to the rubber-tyred wheel. Electrical power is delivered via a cableform that wraps around the unit and doubles back on itself as the truck steers.

Speed controller

The electromechanical speed controller predates the widespread use of electronic motor drives, yet must achieve wide-ranging speed control with limited resources. It must also tend to the needs of the compact traction motor and protect it from aggressive driving. An electronic controller would bring the advantages of better battery economy at low speeds and tighter regulation of creep speed, although the latter can be achieved by a skilful driver with delicate use of the brake. The main principle of operation is progressive resistance control of the armature and field currents; armature control at low speed / high torque, and field control at high speed.

Armature control

Speed controller

Speed controller

Switching on the truck immediately energises the traction motor shunt field at full strength, which remains on at all times. The armature is initially short-circuited to provide electrodynamic braking when the driver’s foot is off the accelerator pedal. Pressing the pedal works a set of cams on the controller by means of a Bowden cable. As the cams rotate they operate microswitches that energise power contactors handling the 100 amp main circuit. The first cam starts the motor by engaging whichever direction changeover contactor is selected by the ‘gearstick.’ At this time there are two high-power resistances in circuit limiting the armature current to a few tens of amps; these are located in one of the chassis ‘legs’, which gets quite warm to the touch after an hour's work at low speeds. The next cam operates the accelerating contactor to bypass one of the two resistances. The third cam operates the full-speed contactor that bypasses the second resistance, leaving the armature directly connected to the battery so that the motor can deliver full torque at moderate speed. To prevent abusive shock-loads on the motor and gearbox there is a mechanism in the controller to limit the speed at which the cams can rotate. If the driver floors the pedal suddenly, the actuating spring must compete with a viscous-fluid damper unit that regulates the rotation, engaging the contactors one by one so that the motor has a chance to build up some speed and back-EMF before each step of resistance is cut out. The direction contactors are electrically interlocked so that the driver cannot reverse direction with the accelerator pedal down and the truck travelling at speed.

Field control

Depressing the pedal beyond the third camswitch step brings the field resistance into play. This is a cam-operated carbon-pile rheostat that is normally spring-loaded into its compressed (lowest resistance) position to give the motor full field. This offers maximum torque for minimum armature current when creeping or accelerating. Once the truck is travelling steadily with all the armature resistance out, a cam progressively relieves the spring-load from the pile, weakening the field and speeding up the motor as the accelerator is pressed further. A fixed resistance is wired in parallel with the pile to guarantee the necessary minimum field current to prevent the motor overspeeding. If the pile were controlled only by the accelerator pedal, excessive armature current would arise if the driver were to depress the pedal fully whilst climbing a gradient, because the field would be weakened regardless of the heavy torque required. To regulate the armature current, a load-sensing solenoid is built into the carbon-pile assembly to give a degree of feedback to the controller. The greater the armature current, the more the solenoid assists the spring in compressing the pile to increase the field current. Heavy load on the motor therefore causes the controller to boost its torque at the expense of speed, behaviour similar to that of a series-wound motor. A further coil is wound on the solenoid to sense the battery voltage and pre-correct the pile resistance for unavoidable variations in shunt-field current. For example, when the battery is low the shunt field will be weaker for a given resistance and the pile must be compressed more heavily to compensate. If, despite these measures, the armature current rises to an unacceptably high level, such as when negotiating a threshold step or starting off out of a rut, an independent overload relay operates, switching the two sections of the shunt field winding from series into parallel and bypassing the carbon pile. This increases the field yet further, boosting the torque to the motor's absolute maximum (which it is rated to withstand only briefly) to nudge the truck over the obstacle. Switching the highly inductive field windings calls for spark suppression to protect the relay, provided by selenium rectifiers in the controller.

The hydraulics

Controller and pump

Controller and pump

Inside pump motor

Inside pump motor

As usual on fork-lifts the load is handled by hydraulic rams, which are more compact than electric drives for long linear movements carrying dead weight. Rams are provided for lifting, reaching and tilting, under the control of spool valves worked by the control levers. The hydraulic system is powered by a 6.3 horsepower electric pump which runs whenever any of the levers are operated (with the exception of lowering which takes place under gravity). Because it is never necessary to operate at full power continuously, the pump motor is short-term rated. This means that while it is capable of delivering 6.3 hp it is only permitted to do so for 15 minutes in every hour without exceeding its maximum temperature. With this limitation imposed the motor can be made smaller than would be a continuously-rated type, an important factor given the compact dimensions of the reach truck. At 24 volts the 4-pole motor draws around 200 amps, so it has heavy brushgear for a motor of its size. It is compound wound and runs as such (at constant speed) when tilting, reaching or lifting slowly. If the lift control lever is pulled to maximum, a contactor cuts off the shunt field. The motor then runs as a series machine, adapting its speed to the weight being lifted and speeding up conveniently if the forks are unladen. An interesting comparison can be made between the size of the electric motor and the size of the hydraulic pump of equal rating that it drives. Hydraulic drive components such as rams, motors and pumps have advantageous performance / size ratios over electric motors and solenoids but it is important to notice that a hydraulic pump is not actually a transducer, i.e. it does not convert energy from one form to another, unlike an electric machine which converts electrical energy to or from mechanical. It is instead a mechanical linkage using a fluid medium as one of its moving parts.

The battery

Battery box

Battery box

The battery consists of twelve lead-acid cells of 680 amp-hour capacity, six in each box either side of the mast, giving 24 volts with a sustainable discharge current capability of many hundreds of amps. They are vented wet-cells which must be regularly topped up with distilled water as the regular fast charging causes rapid evaporation; Although the chemistry is identical to that of a car battery, traction batteries are built with a more robust internal construction to withstand the rapid charge-discharge cycles to which they are subjected daily. Nevertheless they have a finite life and it is likely that Nellie has had a few batteries over her 42 year career. The present set was obtained second-hand from a written-off truck; it is very lively and capable of unloading and racking up a full-weight articulated lorry-load without flagging. For extended periods of use there is a facility for self-unloading the battery boxes by means of arms that connect to the fork carriage. A new set can be picked up from the charging bay for working a second shift, while the first shift’s batteries are charged off the truck.

Semi-automatic charger

The battery is charged by a transformer / rectifier set with a maximum output of 87 amps. The metal rectifier is located in the bottom of the cabinet and the transformer in the top. This arrangement is standard despite the weight of the transformer, because it keeps the rectifier as cool as possible. Although the charger is not ‘intelligent’ and does not adjust the charge rate to suit the initial state of the battery, it does have a voltage detection relay and a timer to ensure the battery receives the correct charge. When first connected to the charger the battery begins charging at the maximum rate. As the state of charge increases the current begins to fall and the battery eventually reaches a point on its charging curve where the voltage is rising quickly and the state of charge is very predictable. The sensing relay in the charger is adjusted to operate at this voltage in order to start an electromechanical timer. Sufficient time is then given for the battery to reach just over full-charge, allowing for the steady fall in charging current that will occur, at which time the charger switches off automatically. This system is preferable to switching off at a predetermined voltage because by the time the battery is fully charged there is hardly any voltage increase with which to detect the true end-point, and the voltage itself is influenced by battery temperature. Naturally the charge-rate, trip voltage and timer duration must be carefully set to match the particular type of battery in use. It is recommended to equalise the cells periodically, meaning to charge slowly beyond the normal fully-charged condition, enabling any cells that have not quite kept up with the others to reach full charge. A position on the charger power switch allows for this by bypassing the timer and reducing the charge rate, using an alternative tapping on the current-limiting transformer.

Charger timer

Charger timer

Charger rectifiers

Charger rectifiers

Charger in use

Charger in use

Finally, after all this technical discourse, I would just like to mention that the horn has a lovely mellow tone that I forgot to demonstrate in the video.

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