Hydraulic pumps

Nov. 24, 1959

G. L. HARRISON

2,913,985

HYDRAULIC PUMPS

Filed June 25, 1958

Sheets-Sheet 1

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Geoffrey L. Harrison

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Nov. 24, 1959 g. l. harrison 2,913,985

HYDRAULIC PUMPS Filed June 25, 1958 2 Sheets-Sheet 2

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Inventor

Geoffrey L. Harrison

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Attorneys

United States Patent

2.913,985

Patented Nov. 24, 1959

2,913,985 HYDRAULIC PUMPS

Geoffrey L. Harrison, Scarborough, Ontario, Canada, as- ® signor to Dowty Equipment of Canada Limited, Ajax, Ontario, Canada, a Canadian company

Application June 25, 1958, Serial No. 744,537

Claims priority, application Great Britain June 25,1957 ¹⁰3 Claims. (CI. 103—37)

This invention relates to hydraulic pumps which are ₁₅intended to generate hydraulic pressure of a selected value and to deliver hydraulic liquid until such pressure is attained, after which the volume of delivered liquid will drop to a value merely sufficient to maintain the selected pressure. The simplest way to accomplish this result is ₂₀to provide an ordinary fixed displacement pump with a by-pass or relief valve having an adjustable loading spring whereby the valve will open when the liquid pressure is attained. With such a simple arrangement, when the desired pressure is attained the full pump output is ₂₅by-passed at pressure and in consequence the prime mover driving the pump is still required to do a considerable amount of work, all of which is converted into heat.

The object of the present invention is to provide a 30 very simple hydraulic pump which s capable of delivering liquid up to a selectable hydraulic pressure and in which, the rate of displacement of liquid is considerably reduced when this pressure is attained so that the hydraulic liquid is not unduly heated whilst maintaining 35 the pressure.

In accordance with the present invention a hydraulic pump comprises a large and a small piston located in large and small cylinders and reciprocated by rotation of a single shaft, to deliver pressure liquid to a hydraulic 40 cutout, a spring loaded valve with adjustable spring loading being provided which is sensitive to the hydraulic output pressure and which is moved by such pressure when it reaches a value determined by the said spring loading to connect the output of the large piston and cylinder 45 to a zone of low pressure. The spring loaded valve also opens a by-pass from the output of the small piston and cylinder at a slightly higher pressure than that at which the large piston and cylinder is by-passed.

One example of the invention will now be described 50 with reference to the accompanying drawings in which Figures 1 and 2 are cross-sections through the pump taken perpendicularly to one another on the respective section lines 1—1 and 2—2.

The example of the invention is for use as a hydraulic 55 pressure generator in a hay baling apparatus. The complete pump is enclosed in a hydraulic oil reservoir 1, a single shaft 2 extending for connection indirectly to the prime mover operating the hay baler. Inside the reservoir 1 a cylinder block 3 is located which extends 60 to the shaft 2 and in which bearings 4 for the shaft are located. The shaft carries an eccentric 5 around which an eccentric ring 6 is located. This ring is connected pivotally at 7 to one end of a large piston 8. The large piston 8 reciprocates in a large cylinder 9 formed in 65 the cylinder block 3. From the opposite end of the large piston a smaller diameter piston 11 extends, coaxially formed and located in a smaller diameter cylinder 12. A port respectively 13 and 14 is formed in the wall of each cylinder adjacent to the position of the 70 included piston when it is withdrawn to its greatest extent. These ports form inlets into the cylinders. At the end of each cylinder remote from the eccentric, an output port respectively 15 and 16 is located, a non-retum valve respectively 17 and 18 being included in each port. These two output ports are connected through pipe 19 to a hydraulic pressure connection 21 located on the wall of the reservoir 1.

Also in the cylinder block a piston valve is located comprising a long small diameter cylinder 22 bored in the cylinder block which contains a simple piston valve member 23 having a single groove 24 formed centrally thereof. One end of the piston valve member projects from the block and is engaged by a helical compression spring 25 which extends towards the upper part of the reservoir. The spring is there connected to a screw mechanism 26 which is adjustable by a manually-rotated knob 20 arranged outside the reservoir. A pair of ports 27 and 28 enter the long cylinder 22 at the position of the groove in the piston valve member, port 27 being in connection directly to the output 15 of the large cylinder and the port 28 being connected directly to the reservoir. The arrangement normally is that outlet 15 from the large cylinder 9 feeds into the groove 24 whilst the piston valve member 23 closes the port 28 leading back to the reservoir. The end of the piston valve member 23 opposite to the adjustable loading spring 25 is exposed through passage 29 to the output pressure as delivered at the hydraulic output connection of the pump, and this pressure will normally act to urge the piston valye member 23 to compress the adjustable loading spring 25. A further port 31 is provided to begin to vent the hydraulic output pressure back to reservoir when the piston valve member 23 is moved a sufficient distance against spring compression to cause full bypass of the output of the large cylinder back to reservoir through port 28.

When in operation on a hay baler the pressure connection 21 on the wall of the reservoir 1 is connected via. a single line to a displacement-type hydraulic ram. This ram is connected to a pair of rails which maintain pressure against two sides of a bale of hay or straw. The side pressure of the rails against the hay or straw needs to be adjustable by the operator in accordance with the weight of bale required, and the condition of the straw or hay at time of baling. The adjustment is made by the operator turning the knob 20 on the reservoir 1 and observing the pressure shown on the gauge 32 provided. During initial movement the whole output of both cylinders in the pump is fed into the jack to move the rails to an appropriate position. When the pre-selected pressure, as determined by the setting of the knob 20, is reached the valve 23 moves and the output of the large cylinder 9 is fully by-passed into the reservoir whilst the output of the small cylinder 12 is by-passed wholly or partially through port 31 to maintain the desired pressure in the jack. The amount of oil that is pumped at pressure back to reservoir 1 is therefore only the output of the small piston 11 and cylinder 12 and the heat developed is comparatively small. When an increase in pressure is required the whole output of both cylinders is available for moving the jack as quickly as possible to its new position. The system is in balance when the outward force exerted by the bale of hay or straw equals the inward force exerted by the hydraulic ram. If a change in the outward force exerted by the bale occurs the balance is upset and the hydraulic ram extends or contracts until the system is again in balance. If the outward force exerted by the bale decreases, the pump pressure drops momentarily, thereby closing the pump by-pass passage or passages 28 and 31, and oil flows from the pump to the ram until the by-pass pressure pre-selected by the operator is restored. If the outward force exerted by the bale in2,913,985 creases, the pump pressure rises momentarily, thereby increasing the opening of the pump by-pass 31 and causing oil to flow from the ram to the reservoir until the by-pass pressure pre-selected by the operator is restored. The ram is therefore comparable to a spring of zero rate. This ensures that if the length and the height of the bale is fixed the width of the bale may vary without any appreciable change in the outward forces exerted by the bale. This allows the volume of the bale to change and compensate for local variations of hay or straw density, thereby ensuring that bale weight remains substantially constant.