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FAQnewred.gif (906 bytes)           

Section 1.13 - Hydraulic System Hand Pumps

hrule_bl.gif (260 bytes)

Functions

  1. Hydraulic system hand pumps are used to test the hydraulic system when the plane is on the ground; and,
  2. acts as an Emergency system of power

Principles of Operation

The hand pump converts the power of a human being to hydraulic horsepower. For this reason, such pumps are used in older model aircraft.

Types

There are several types of handpumps.

  1. Single action - single impulse (S.A.S.I.)
  2. Double action - double impulse (D.A.D.I.)
  3. Single action - double impulse (S.A.D.I.)

Analysis of a S.A.S.I. Pump

The pump works on the principle of mechanical advantage
When pilot moves the handle away to L (see diagram below), a low pressure is caused to form in the chamber of the pump, Z. Since the reservoir liquid pressure is greater than the pressure in Z, liquid is forced around the check valve from the reservoir line. When the pump handle is moved to R, a positive pressure will form in Z, causing the check valve to the reservoir side to close. Since the hydraulic system liquid is incompressible, it is forced out through the bottom check valve to the system line.

Suppose the force delivered by the pilot was F1 = 100 lb, at a distance D1 = 20 inches from the pump handle pivot, and suppose that D2 = 1 inch from the pivot to the pump piston.  Then by moment equilibrium (F1D1=F2D2), the force acting on the piston would be F2 = 2000 lb. If the piston area on which the oil acts is A = 2 square inches, then the pressure developed is Eq024.gif (384 bytes)

hydro035.gif (10760 bytes)

The volumetric output Eq025.gif (267 bytes). In this case, Eq026.gif (453 bytes). But a man’s average output is about 50 lb for F1. Therefore, the piston area must be made half its present size (if the pressure is to remain constant). Thus, volumetric output will then decrease to 1 cu. in., since Eq027.gif (496 bytes).

Problem: (SASI Pump)
hydro038.gif (1865 bytes)Given a SASI pump connected to a hydraulic system under 3000 psi pressure and the mechanical advantage of the pump (the ratio of D1 to D2 ) to be 30 to 1, find:
1)  the number of cycles needed to pump 300 cubic inches of fluid into the system; and,
2)  the force to be applied at the pump handle. Assume that the piston surface area is 0.5 square inches.

Since the system pressure = 3000 psi and this acts on a surface area of 1/2 square inch, the force generated by the hydraulic fluid is 1500 lbs.

Using the mechanical advantage of 30 to 1, we find that the force at the pump handle will be

Eq028.gif (322 bytes) or Eq029.gif (362 bytes).

Since the piston stroke is D2, the volume moved per cycle is D2 times the piston surface area

Eq030.gif (360 bytes)

or

Eq031.gif (343 bytes)

Thus, per cycle, 0.5 cubic inches of fluid is pumped into the system.

The number of cycles needed to pump 300 cubic inches of fluid may be found from

Eq032.gif (709 bytes)

then

Eq033.gif (668 bytes)


Analysis of a D.A.D.I. Pump

The DADI pump works in the same way that the SASI pump works, except that while chamber A is filling up from the reservoir, chamber B is pumping oil to the system.

hydro034.gif (5717 bytes)

Analysis of a S.A.D.I. Pump

As the pump handle pulls the piston to the left, the oil trapped in A is pushed out through check valve 1 to the system. At the same time, a low pressure is created in B and ball 2 opens, letting in oil. As the pump handle pushes the piston to the right, a positive pressure is created in B which closes check valve 2. But, since oil in B is trapped, it escapes to side A when the pressure is great enough to open check valve 3. Now that the oil is in side A, the pressure on that side is still great enough to open check valve 1.

hydro036.gif (6073 bytes)

 


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Updated: March 12, 2004