Geared Pumps

Revision: 8/OCT/2024.

Mini excavators use a fixed displacement hydraulic pump operating in an Open Center Hydraulic Circuit. In an Open Center Circuit the pump turns and the flow starts immediately, which is then directed back to the tank through a central passage within the directional control valve. When one of the directional control valve’s spools is stroked, the flow is sent by the control valve to the respective motor or cylinder and pressure is created. Once the pressure exceeds the load, the load moves and the motor turns or the cylinder is actuated.

The hydraulic oil flows from the tank to the pump on the inlet port. The gears push the oil to the other side on the outlet port and it is circulated into the system. The only job of the pump is to sustain a rate of flow (displacement). The pressure on the system is created when this flow encounters a resistance, motor rotation, cylinders and the respective load. The system design and its components is what actually dictates the pressures of the hydraulic system.

The inlet port is called the “Suction” or “Low Pressure”. Anything connected on that side is said to be on the “Suction Side” or “Low Pressure Side”. The outlet side is called the “Pressure Side”. The inlet port is the largest port on the pump and the outlet is the smallest.

As you may have already noticed, the flow always goes in only one direction. From the factory a pump is designed to either rotate to the left (counterclockwise) or to the right (clockwise).

Right (Clockwise) Rotation	Left (Counter clockwise) Rotation
Inlet (Largest Port) on left side and Outlet (Smaller Port) on right side .	Outlet (Smaller Port) on left side and Inlet (Largest Port) on right side.

Pumps used on 1 ton mini excavators are right or clockwise rotation which is usually denoted by an “R” on the product description.

There are five important aspects of pumps, the displacement rate or flow, the mounting plate, the type of shaft, the direction the shaft must rotate, and the type of hydraulic oil ports used.

Displacement and Flow

These pumps are rated by their displacement force (flow) measured in cubic centimeters per revolution (cm³/rev) or in mL per revolution (both units are equal) (for us in the US we can think of Gallons per Minutes as an analogy.) The flow may also be listed in “CC” which is just another way of representing cm³, which is also written as “ccm”, but all of these units are equivalent.

Pumps are grouped by their displacement rate. We have groups 1, 2 and 3. In Group 2 we have pumps from approximately 4 cm³/rev to 25 cm³/rev displacement. All 1 ton mini excavators will have a Group 2 gear pump.

A pump will have a nominal displacement flow and a maximum displacement flow. Since this flow is produced by each revolution the flow always goes in tandem with a given RPM produced by the excavator engine.

Remember a nominal value is the value that a component is designed to work constantly and the maximum is the value that it can withstand for brief periods of time.

The displacement rate of a gear pump is generally computed at 2500 rpm. A pump rated at 16 cm³/rev will sustain that flow when operated at 2500 rpm. WARNING: NO all manufactures compute their flow rate at 2500 rpm, and sadly most manufacturers of generic white label pumps will not tell you how they rated their pumps.

Usually 2500 RPM is the nominal RPM for many pumps in this group. The maximum RPM can be from 3000 RPM to 4000 RPM.

Pressure

Let's recall that pressure on the system occurs when flow meets resistance inside cylinders and motors. In the mini excavator the only job of a pump is to sustain a rate of flow at a given nominal RPM.

The pressure rating on a pump is strictly the max pressure it can withstand (nominal and max), the resistance it encounters will dictate the actual pressures exerted on the system and these pressures are variable.

The nominal pressure is the system pressure that the pump was designed to operate under. The pressure can be measured in many units but mini excavators will use Pascals or MegaPascals. A 25MPa pump can withstand outbursts of pressures up to 25 MegaPascals or 250 Bars (1MPa = 100 Bars, 1 MPa = ~145 PSI). The nominal pressure is seldom listed on the pump’s label but it should be around 5MPa less, so for example a 25MPa operates at a nominal rate of around 20MPa.

Shaft and Mounting Plate or Flange

The mounting plate or flange dictates how the pump is mounted in the machines. There are many types of mounting plates but thankfully for 1 ton mini excavators we are looking at a two screws oval mount or a 4 screws square mount.

The common mount used is a two screw oval mount. The important measurement here is the distance between the screws. While this mount has many variations the screw distance must be 106 mm or 4.187 in. In practice these pumps come with open ends for the screw holes making them far more compatible with different setups and tolerances.

The mounting of pumps are defined in a couple of industry standards. The first standard is SEA “A” flange formerly defined in the SAE J744, “HYDRAULIC PUMP AND MOTOR MOUNTING AND DRIVE DIMENSIONS”. This standard is also known as ANSI 82-2 and you may find it described as “SAE 2 bolt A ANSI 82-2”. A SEA “A” flange has the screw holes with a distance of 106mm or 4.187in from center to center.

In this standard we also find the shaft. The common shaft is a 9 teeth spline shaft (30° involute spline). The diameter of this shaft is about 15 mm and it connects with a motor shaft using a coupling sleeve commonly a plum blossom coupling.

Hydraulic Ports

Common pumps found in mini excavators generally use BSPP or Metric fittings where BSPP fittings are the common ones.

	A port with 4 screws is called a flange port. Screws arranged in a square is a German Flange. Screws arranged in a cross is a European Square Flange. Ports without screws are either Pipe threads (BSPP) or Metric threads.
Metric Threads A metric flange uses o-rings and the matting thread has a rim for the o-ring. This thread has a shallow cone throat.
	Common port sizes
	Outlet	Inlet
	M18 x 1.5	M22 x 1.5
	M27 x 2	M33 x 2
Pipe Threads (BSPP) The BSPP is the thread that you are more likely to find on a mini excavator. Notice that a BSPP has a deeper cone throat, the thread is coarser and it lacks the rim of the o-ring we see on metric threads. The case drain ports are commonly G1/4.
	Common port sizes
	Outlet	Inlet
	G1/2	G3/4
	G3/4	G1