SHUTTLES BOOST EFFICIENCY

The quick fix for pump efficiency

Improving fuel economy of hydraulic systems can now easily be realised by increasing pump efficiency.
Innas developed a simple solution to increase pump efficiency: so called Shuttles diminish commutation losses. Implemented in a standard swash plate pump, efficiency peaked at 96,4%, an increase of 4,5%!
Shuttles are universal and can be implemented in all piston pump types, such as bent-axis, swash plate, and radial piston. In an average excavator, the 9 ceramic balls in a pump can save over 1000 liter diesel per year.

MEASUREMENTS

Testing shuttles in a conventional Rexroth A4FO28 led to remarkable results: the peak efficiency has increased from 91,9% to 96,4%
The average efficiency has increased with 3,5%
At lower speed (below 1000 rpm), the total efficiency increased with more than 30%

The figure on the left shows the measured efficiency increase after implementing shuttles in the pump's field of operation.

PRINCIPLE

Continious (de-) compression

In hydraulic pumps and motors, the pressure alters continuously between low and high pressure levels. In, for instance, an axial piston pump, the pressure level in each cylinder changes two times per revolution, once in the top dead center and once in the bottom dead center. In case of a pump with nine pistons running at 3000 rpm, the pressure changes 900 times per second.

Compromise

The pressure relief grooves are meant to soften the pressure change during commutation, but these grooves are a rather poor compromise, since the dimensions of the grooves can not change with the operating speed and pressure. The situation becomes worse, now that the new trend, towards electro-hydraulic applications, demand the pumps to run in a much wider speed range.

Cavitation & Hammering

Pressure relief grooves act like a resistance, creating a connection between the barrel cylinder and a high or low pressure kidney. It operates basically as a restricted short-circuit. The compression and expansion is very fast with a very high rate of pressure change, almost like a hammer stroke. This constant banging, in the mentioned example 900 times per second @3000 rpm, not only causes high noise levels, but it is also a source for cavitation and for energy losses.

Portplate without grooves

Instead of short-circuiting, the oil in each cylinder can also be compressed or expanded by means of the piston movement itself. If, for instance, the oil needs to be compressed in the bottom dead center, the compression can start as soon as the piston has passed the bottom dead center. After that, the cylinder does not have a connection to any of the kidneys of the port plate, at least as long as the piston movement is required to compress the oil to the desired next pressure level.

With this design, the pressure rise will be much softer than with pressure relief grooves, and cavitation and energy losses due to commutation will be avoided.

Shuttles between cylinders

But, as with the pressure relief grooves, also in this case the port plate can only be designed for one operating pressure, for instance for the maximum pressure level of the pump. Without any extra changes in the design, the commutation would result in strong pressure overshoots and cavitation if the pump is not running at maximum pressure.
This is why shuttles are needed. The shuttles open a connection between the cylinder and the next kidney as soon as the required pressure level in the cylinder is reached. They allow the pump or motor to have an almost ideal commutation, without any commutation losses. With the new port plates and the shuttles, the commutation is governed by the piston movement, which results in a much softer compression and expansion.

Lightweight Ball

A shuttle is a small cylinder with a ball inside. At both ends of this volume there is an opening through which oil can flow in and out. Between these two ends the ball can move. In the end positions, this ball closes thereby preventing any flow leaving the shuttle. The shuttles are small and lightweight. The dimensions are typically a few millimeters in diameter and length.

The shuttles only work in a correct way if the balls are in the correct position –at the right side of the shuttle cylinder– whenever a commutation starts. For this reason the shuttle connects to the pump cylinder on the one side, and at the front of the barrel on the other side. When in operation, the barrel port creates a pressure differential which is ‘sensed’ by the shuttle and forces the shuttle ball each time in the desired position, both in the top and bottom dead centers.

NEW PORT PLATE

Simplified port plate

Shuttles are to be combined with a new port plate design. This design is far more simple.

It has larger sealing lands which avoid the considerable commutation losses.

The pressure relief grooves, which only work optimal at a certain pressure, are no longer required. Pressure overshoot and cavitation due to the relieve grooves is avoided.

Commutation is governed by the piston movement itself in the larger seal lands, and shuttles realise a soft compression and expansion.

DESIGN

The shuttle balls have a low mass and are extremely responsive. There is no spring to position the shuttle balls. There are also no strict tolerance requirements. The balls are somewhat smaller in diameter than the shuttle cylinder in order to prevent any friction between the ball and the shuttle cylinder. During the suction stroke and the delivery stroke the oil pressure on both sides of the cylinder is about equal and leakage is not a concern.

Shuttles are a simple, inexpensive solution for improving the pump efficiency while simultaneously strongly reducing the noise levels of hydraulic pumps and motors. They can be applied in axial piston pumps and motors, but also in radial piston pumps and motors, or in any other pump or motor with clearly defined commutation by means some sort of porting.