Kawasaki’s new closed circuit products: overcoming technological challenges
Kawasaki’s hydraulic equipment business has set out toward its next centennial. A global leader in hydraulic equipment for excavators, the company is now seeking to build on its success by making inroads into new fields, with new products.
Open Circuit vs Closed Circuit
Hydraulic pumps that provide the energy to drive motors and cylinders through hydraulic pressure come in two types: one for an open circuit, and the other for a closed circuit. While hydraulic excavators employ an open-circuit system, a continuously variable hydrostatic transmission (HST) consists of a closed-circuit system.
In an open-circuit system, hydraulic fluid leaves the tank and flows through a pump, valve and motor (cylinder) before it returns to the tank. In a closed-circuit system, on the other hand, hydraulic fluid is circulated between a pump and motor (see Fig. 1). Although a closed circuit is simple in structure, the pump must be fitted with various features, such as a charge pump for replenishing hydraulic fluid.
The biggest market for closed-circuit hydraulic equipment is industrial vehicles provided with an HST drive system, which include wheel loaders, tractors, and forklifts. As such, motors used in a closed-circuit system require high speed and high torque capabilities, as well as a pump for generating force.
Kawasaki have developed products specifically for use in vehicles with an HST drive system to enable them to supply to wider variety of industrial applications, including agricultural machinery. The first products to be released in this category are the K8V series of swash plate type HST pumps and the M7V series of swash plate type axial piston motors.
K8V pumps supply hydraulic fluid at a pressure of 40 MPa. They not only boast the world’s top-level pump efficiency, but also achieve low noise and high reliability, responding to the needs of industrial vehicles.
M7V motors receive the force of hydraulic pressure generated by a K8V pump to produce high speed and high torque. Compared to a motor for hydraulic excavators, which is capable of operating at approximately 2,000 rpm, a motor for HST can run at 5,000 rpm, nearly doubling the power density (power per unit weight).
Technological challenges of closed-circuit systems
Masaki Ohnishi, Senior Manager of the Engineering Division of Precision Machinery Company, explains the technological challenges of these products as follows: “By increasing speed and torque, the amount of heat generated will inevitably increase as well, which means that sliding components where parts rub against each other must use materials with greater heat resistance. So our job begins with choosing the right materials, enhancing the dimensional accuracy of parts, and increasing surface hardness. We also need to adjust the balance of hydraulic pressure to ensure the shaft remains steady when rotary components such as pistons and a cylinder block rotate.”
Despite these challenges, the K8V series achieves the world’s highest pump efficiency and low noise. To increase efficiency, both the clearance between the cylinder block and pistons and frictional force were minimised. By using highly heat-resistant piston materials to reduce the clearance, the amount of hydraulic fluid leaking from the clearance can be reduced. This will enable the pump to supply hydraulic fluid more efficiently, and high power can be ensured even when the pump is reduced in size.
To reduce noise, the shape of the valve plates that control the timing at which pistons take in and discharge oil was re-examined. Noise is generated as a result of the pressure change that occurs inside the piston chambers. Thanks to the newly redesigned valve plates, the timing in which hydraulic fluid is taken in and discharged was optimised.
In order to optimise hydraulic pressure balance, the pistons, cylinder block, and other rotary components were fully redesigned.
There were also technological challenges to overcome in the development of the M7V motor. Kouichi Komada, who was in charge of development, says, “Because HST travels at 40 to 50 km/h, 5,000 rpm was a prerequisite. But the significance of this challenge was in achieving this with a swash plate type motor.”
As with the pump, a high-speed, high-pressure motor is subject to high heat. For this reason, materials with high resistance to heat and pressure were chosen, and the surface of the materials was heat-treated.
Komada explains, “To optimise hydraulic pressure balance, we fully redesigned the pistons, cylinder block, and other rotary components, and thoroughly re-examined the shape and other aspects down to the smallest detail.”
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