Tuesday, April 5, 2011

Designing the Hydraulic Fluid Power System for the Selective Asparagus Harvesting Machine

The harvester utilizes hydraulic power to run the air compressor motor, the conveyor motors, the pickup roller motors, the alternator, and the header lift cylinders.

I will be using a pressure compensated pump to provide the hydraulic fluid that powers all of the above items. That way I can use simple flow controls to control the speed of each of the aforementioned items. The pump only puts out as much hydraulic fluid as required by the load. If you were to shut off the flow controls the output from the pump would drop to zero.

I will use a PTO pump hanging off of the PTO shaft on a tractor to drive a motor that in turn drives the pressure compensated pump.

Let’s determine the horsepower, pressure, and flow rates that will be needed by the various components. I would like to keep the system pressure below 1,500 psi.

Pickup Roller and Conveyor Motors

The hydraulic motors used to power the pickup rollers and the conveyors are all identical Char-lynn hydraulic motors with a displacement of 1.95 cubic inches per revolution. I’ll be running the conveyors and pickup rollers at around 100 rpm. That’s about 12 inches per second for the conveyors.

At 100 rpm the motors require about 1 gallon per minute of hydraulic fluid. I’ll be running the three pickup roller motors in series and the three conveyor motors will be in series. That means I will need 1 gpm for each of the two circuits.

However, to be on the conservative side, I will use a figure of 2 gallons per minute for each of the two circuits. So far… 4 gallons per minute needed. Very low horsepower though. The pressure needed for any individual motor is probably only 200 – 300 psi. That is why I am running 3 motors in series. If I keep the system pressure below 1,500 psi I won’t need case drains on the motors.

The next item to consider is the motor that runs the compressor. I’m replacing the electric motor on an air compressor with a hydraulic motor. The motor will be controlled with a hydraulic solenoid valve just as the electric motor was controlled with a pressure switch. Now the pressure switch will activate a solenoid valve to run the compressor motor when the air press drops to less than 150 psi. The switch shuts the valve off at 175 psi.

The motor is a 7.5 horsepower motor and runs the compressor at about 900 rpm through a belt and pulley arrangement. I will be using a 2:1 ratio so I will want the hydraulic motor to run at 1800 rpm to keep the compressor at 900 rpm.

If I choose a motor with a displacement of about 1.25 cubic inches per revolution the motor would require about 10 gallons per minute. At 8 horsepower it would require about 1,500 psi.

So now I have 4 gpm + 10gpm = 14 gpm.

The last item is the header lifting mechanism. The two lift cylinders that provide the lifting power for the header are 2 inch bore cylinders. With a system pressure of 1,500 psi each cylinder will lift over 4,000 pounds and the header only weighs about 1,000 pounds. There are two modes of operation for the lift cylinders.

One mode is when normal harvesting is underway. In that mode the lift cylinders are operated by the”slow” direction control valve. The slow valve has flow controls installed and limit the flow so that the header will only change height at about ½ inch per second. For 2 inch bore cylinders plumbed in parallel that will require less than 1 gallon per minute.

The other mode of operation is when the header must be raised quickly because of a cylinder fault. We would like to have the header lift up quickly enough to prevent binding of the cutting cylinders. That means we need to lift it about 24 inches in about 2 seconds. A flow of 8 gallons per minute into a 2 inch bore cylinder produces a speed of about 10 inches per second. So ideally we would like to have a flow of 10 gallons per minute.

This mode of operation will hopefully rarely occur if ever. The extra 10 gallons a minute would require much larger pumps, motors, and hoses too. What I will do is have the compressor motor shut off when the fast header up command is encountered. If the header is being raised because of a cylinder fault or because the driver is going to turn around at the end of the row, then the compressor doesn’t need to be running. I can program that easily into the controller chips on the circuit boards.

That leaves me with the 14 gallons per minute plus 1 more gallons per minute for the lift cylinders bringing the total needed maximum flow rate from the pressure compensated pump to 15 gallons per minute at 1,500 psi or less.

For now I will select a pump with a displacement of 1.71 cubic inches per revolution. (A Bosch AV10VSO Pump - size 28). To produce the 15 gallons per minute I will need to run the pump at about 2,000 rpm. The maximum rpm for this pump is 3,600 rpm.

Now I need to select a motor to drive the pressure compensated pump. Since I need to run the pressure compensated pump at about 2000 rpm for the maximum needed flow, I will select a motor that will spin at 2,000 rpm with a flow of about 17 gallons per minute. A motor with a displacement of 2 cubic inches per revolution will work nicely.

The last size choice is for the PTO pump that will hang off of the PTO shaft on the tractor. I’ll be using a Prince Model HC-PTO-9A which produces a maximum flow of 21 gallons per minute at 2,000 psi. When the tractor is at full throttle the PTO shaft rotates at 540 rpm. That is the rpm the pump will put out 21 gallons per minute at. Since the PTO pump is a positive displacement gear pump it will force 21 gallons a minute out when running at 540 rpm, so the drive motor for the PC pump will have to handle that flow without exceeding its own maximum rpm rating and also without exceeding the PC pumps maximum rpm rating. At 21 gpm the drive motor I selected will spin at about 2,600 rpm which works for both the motor and the pump.

Hydraulic Reservoir Selection

A commonly followed rule of thumb for selecting a reservoir size is to use the same size reservoir in gallons as the pump puts out. In my case I am using a much larger reservoir, about 50 gallons. It fits nicely on the machine and it will provide a lot more heat radiation area to keep the oil temperature down.

Hose Selection

Selecting the right hoses is important. The suction lines have to be big enough to prevent cavitation in the pump. But as the hose diameter increases the costs go up fast. Typically the hydraulic flow through hoses should be kept below about 15-20 feet per second for pressure lines and oil return lines and below 4 feet per second for suction lines for the pump inlets.

The highest flow for my system will be the 21 gallons per minute going into and coming out of the PTO pump. A 1-1/2 inch diameter suction hose from the tank to the PTO pump is a good choice producing a flow velocity of a little less than 4 feet per second at about 22 gallons a minute. For the pressure line from the PTO pump to the drive motor I can use 1” I.D. hose and stay below the 20 ft/sec.

The PC pump will use the same size hoses for the inlet and for the outlet to the pressure manifold connection as the PTO pump, 1-1/2” suction and 1” pressure lines. The next largest flow is from the manifold to the compressor motor. A ¾” line will be fine for that connection and a ¾” line from the motor to the reservoir. The lift cylinders will only rarely have high volume flows so we can undersize the lift cylinder hoses. I’m going to use ½” hoses for the lift cylinder plumbing. For the conveyor and pick up roller hydraulics I will use 3/8” hoses.

Hydraulic Valves

There are three control valves and two separate flow control valves in the system. The two flow control valves are in series with the conveyor motors and with the pickup roller motors. They are used to set the conveyor and pickup roller speed.

There are two closed-center directional control valves that control the lift cylinders. The valves each have an up and a down solenoid. The third control valve is a solenoid controlled on/off valve for turning the compressor motor on and off.


I’ll use a spin-on type filter on the pressure return lines from the two pumps.

And there you have it… the basic design of the hydraulic fluid power system for the selective asparagus harvesting machine.