US2913985A - Hydraulic pumps - Google Patents

Description

G. L. HARRISON HYDRAULIC PUMPS Nov. 24, 1959 2 Sheets-Sheet 1 Filed June 25, 1958 Fla.

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3244M h-romsvs NW; 9 G. L. HARRISON 2913985 HYDRAULIC PUMPS filed June 1958 2 Sheets-Sheet 2 Geoffre 'L. fl l r zn W i w A-rmaueva United States Patent HYDRAULIC PUMPS Geoifrey 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 r 3 Claims. (Cl. 103-37) This invention relates to hydraulic pumps which are intended 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 suflicient to maintain the selected pressure. The simplest way to accomplish this result is to provide an ordinary fixed displacement pump with a bypass 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 fulljpump output is by 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 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 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 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 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 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 11 and 2-2.

The example of the invention is for use as a hydraulic 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 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 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 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-return 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 valve 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 sufiicient 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 lay-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 increases, 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.

I claim as my invention:

1. A hydraulic pump comprising a large piston and cylinder unit and a small piston and cylinder unit, means to reciprocate said pistons conjointly, an output port from each cylinder and a non-return valve in each thereof, a common hydraulic output passage with which the said output ports communicate, past their non-return valves, input ports to the respective cylinders from a low pressure hydraulic supply source, closed by pressure-inducing movement of their pistons; a spring-loaded by-pass valve comprising an elongated plunger grooved intermediate between its ends and a by-pass valve cylinder wherein said plunger is slidably mounted, spring means urging said plunger in one sense, a first by-pass passage between said valve cylinder, in the vicinity of the plungers groove, and the interior of the large cylinder, and a second by-pass passage between said valve cylinder, at the end towards which the plunger moves under the influence of its spring means, and the common outlet passage; a first relief port spaced in the sense of pressure-induced movement of the plunger from said first by-pass passage and affording communication between the latter, and hence the first cylinder, and a low pressure region with increasing pump output pressure, and a second relief port adjacent that end of the plunger which is subjected to common output pressure, and spaced in the same sense to connect the common output passage at least partially to a low pressure region following opening of the said first relief port.

2. A hydraulic pump as set forth in claim 1, including means to adjust the spring means for opening of the first relief port fully upon attainment of a predetermined output pressure, and for further opening the second relief port upon the output pressure tending to exceed such predetermined pressure.

3. A hydraulic pump as claimed in claim 1 including a reservoir surrounding the pump and by-pass valve and also enclosing a quantity of hydraulic liquid at low pressure to feed the large and small piston and cylinder units and to collect liquid from the by-pass valve.

References Cited in the file of this patent UNITED STATES PATENTS

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