60 Years Ago: Two Pump System Provides Synchronized Motion

High pressure, low slip, radial piston pumps with a gear pump powered stroking mechanism supply equal volumes to each of the two car dumping jack cylinders. The circuit has safety and control features which are required by many oil-powered machines.

Three major design problems are solved in the construction of a hydraulic power system used to dump slag loaded railroad cars. Keeping two cylinders moving together regardless of varying loads, eliminating possibilities of shock in the system because of the large mass being handled, and providing rigid piping on oscillating cylinders are all accomplished in the design.

While these three considerations were of major importance, several other requirements had to be met.  Protection against the outdoors and against dirt had to be provided. Operation had to be simple “up-down” pushbutton control properly interlocked for safety and efficient operation.

Synchronizing the motion of two cylinders is always a difficult problem. Rate of travel depends on the volume of oil delivered to a cylinder, and to move them together requires equal flow of oil to each cylinder. Leakage, pump slip, changing workloads and varying friction loads all affect oil delivery. On the car dumping system, uneven loading is almost inevitable, yet the jack cylinder must move together to prevent twisting of the car body.

Acceleration without shock

These power packages, which supply oil to the cylinders, are connected electrically so that they operate simultaneously.

To ensure synchronization of the jack cylinders, it was decided to power each one from a separate variable and reversible volume pump. The use of such pumps, in addition to offering high volumetric efficiency, allows the necessary incremental adjustment of the volume delivered in order to obtain synchronization.

The variable and reversible volume feature of the pumps offers the additional advantage of permitting smooth acceleration and deceleration of the cylinders without shock. This is accomplished in the following way.

Referring to the circuit drawing, the pumps are normally on negative or idle stroke so that the actual pressure port becomes a suction port and oil is pulled from the tank through the check valve, and circulates back to tank at zero pressure. Pressing the “up” button energizes solenoid A-1 in the main pilot valve, and B-1 in the control valve for the double acting pump stroking cylinder. The 2 inch, four-way two position valve, which provides directional control of the cylinders, shifts to the “up” position; the pumps are stroked from full negative to full work stroke which builds up the oil delivery slowly to accelerate the car. Releasing the button de-energizes B-1, but not A-1, and the pump is stroked back to negative or idle position, decelerating the car to a stop.

To lower the car, which operation was to be carried out at the same speed as dumping, it is obvious that the pumps would have to be placed on something less than full stroke, for they are now delivering to the rod end of the cylinders. As a result of some other considerations, the rod end area was finally set at one half the total piston area, thus calling for the pumps to be placed on only half working stroke for lowering. An actual volumetric capacity of the pumps is 44gpm.

With the pumps on idle stroke after dumping, the “down” button is pushed energizing A-2, B-1, and C-1. The main 2 inch, four way directional control valve now shifts to the “down” position with the pumps once again being stroked from full negative to work stroke for accelerating the car. However, in this case a second stroke positioning cylinder is brought into play through pilot valve C-1, which ahs been energized, so that the pumps can move only to half working stroke.

No Valve Reversal Under Pressure

When the car is completely lowered, the jack cylinders have additional stroke, which allows the hooks on the rods to engage the cam pedestals so that the cylinders are pivoted away from the track to provide adequate clearance for the locomotive and cars.

Releasing the “down” button deenergizes A-2, B-1, and C-1, returning the pumps to idle position.

It can be seen, then, that the main 2 inch, four-way valve never reverses under pressure, because the pumps are put on delivery stroke only when the “up” or “down” button is contacted. In this way, hydraulic shock of flow reversal is prevented.

It is interesting to note that the action of the variable volume circuits. Most such circuits use a variable volume pump to give large volume, large pressure rapid advance, and then as pressure builds up the pump shifts to a short stroke, low volume high-pressure position. During idling, the pump is by-passed to circulate at zero pressure. In the dump car circuit, the pumps are stroked in reverse for idling and are put on full stroke at maximum pressure for the working portion of the cycle.

To provide pilot pressure for stroking the pumps and main valve, each unit incorporates a 3 gmp gear pump operating at 200 psi. A pressure switch in each of these pilot circuits functions as a safety device to prevent the control circuit from functioning unless pilot pressure is established. These switches tie in with the main motor starters so that the main pump motors will start only when pilot pressure has been developed.

Protection is also offered when shutting down in that the main motors stop first and, through the use of a timer, the pilot pump motors will shut down afterwards. Note also the spring loaded check valves on the exhausts of the pilot valves, which provide a 10psi back pressure. These were inserted to make certain that the pilot lines would always be preloaded, avoiding any delay in response of the controls.

The counterbalance valves in the lines to the jack cylinder are provided for positive operation should a full or partially loaded car need to be lowered for any reason, and to prevent over-running of the car due to a sudden shift of the load during dumping.

Two cylinders, each powered by a separate hydraulic circuit, raise this car to dump a load of slag.

The pressure switches in the main circuit serve a two-fold purpose for safety precautions. First, they sense system pressure preventing response of the control relays, which energize the solenoids on the main pilot control valve, until system pressure has dropped due to the pumps going on idle stroke. This means that release of the “up” button and immediate contacting of the “down” button will not reverse the direction of the car until the system pressure has dropped to a value low enough to prevent shock; only then will the main four-way valve reverse and the pumps return to delivery stroke.

Secondly, should the operator fail to release either the “up” or “down” button when the cylinders have reached the end of their stroke, the pumps would normally maintain the maximum pressure of 2500 psi, delivering through the relief valves provided on the unit. However, to prevent this unnecessary waste of power, when the cylinders reach either end of their stroke and system pressure rises, the pressure switches deenergize the pump stroke control pilot valves so that the pumps are placed on idle stroke, whether or not the operator releases the “up” or “down” button.

Double Solenoid Valve For Safety

Safety is also the reason for selecting a double solenoid pilot valve for the 2-inch four-way directional control valve. In order to reverse the direction of the jack cylinders, it is necessary to energize a circuit. If this pilot valve were single solenoid spring return, power failure would cause sudden reversal of the jack cylinders with consequent hydraulic shock. On the other hand, the pilot valves for stroking the pump are of the single solenoid spring return type. In order to cause the pump to deliver oil to the system, these valves must be energized; thus, any electrical power failure would cause these valves to place the pump on idle stroke, stopping the jack cylinders without shock in whatever position they happen to be.

Rigid piping to the oscillating jack cylinders s needed because the 2-inch lines must carry the 44 gpm, 2500 psi oil flow. To permit use of rigid piping the trunnion design of these cylinders uses hydraulically balanced swivel pipe connections. The pipe is rigidly fastened and the cylinder oscillates freely.

All design requirements for this dump car system were met by the application of rugged hydraulic components, which could stand up under long outdoor operation.

L. Arthur Lassman is with Benjamin Lassman & Son Inc., Glenshaw, Pa.


This article originally was published in the September 1954 issue of Applied Hydraulics magazine, the former name of Hydraulics & Pneumatics.

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