Mechanical Technology April 2015

⎪ Hydraulic and pneumatic systems ⎪

solutions

back into the pump. The output shaft is, therefore, always ‘in gear’. If the vehicle starts to run away, the motor tries to feed more oil back to the pump,

but it cannot, because its supply is limited to that coming from the pump. So the accel- eration is immediately arrested,” he explains, adding “the hydraulic circuits become quite complicated on these sys- tems, but this is what we at

The SH11C series motors are bent axis, axial piston hydraulic units with fixed dis-

Axiom are good at – developing drives of all descriptions for vehicles and mobile equipment using variable displacement pump and motor combinations.” Citing cement mixer trucks, he says that variable speed hydraulic systems allow the cement to be turned over at “very low rpm” while transporting ce- ment. “Then, when the truck gets to site, the rotation direction is reversed and the speed is driven up to allow the cement to be pumped out via an Archimedes screw arrangement,” he says. Also: “through the joystick, the vari- able speed hydraulic

placement for operation in open and closed circuits. Pressure up to 430 bar and speeds up to 5 000 rpm are possible. tors for higher speed applications. The Black Bruin motors typically operates at speeds of up to 400 rpm, while SAM motors, at lower torque, can be driven at 2 500 rpm,” he says. And SAM low speed orbital motors? “These are lighter duty motors for low speed, low torque applications such as winches, crane drives, excavator systems and small mixers,” Simpson responds. Linking SAM Hydraulik pumps with the actuators – motors or cylinders – under control, Axiom designs turnkey hy- draulic control circuits based on the use of manifold blocks and Sun Hydraulics cartridge valves. “We have the design capability to quickly design turnkey hydraulic system or to modify ex- isting systems – and we machine our own manifold blocks for each design,” reveals Simpson. “These are then populated with Sun car- tridge valves,” he adds. “This combination has been very good for us and, while our new business has been affected by the mining downturn, we are seeing increased sales of complete units for spares. This tells us that people are using this opportunity to prepare for the next upturn. In addition, we are seeing growth in mobile applications, mostly for diesel-driven units,” Simpson informs MechTech . To cater for current and future de- mand, Axiom Hydraulics is looking for new premises to double the size of the existing machine shop, from around 2 000 m 2 to 4 000 m 2 .“We are currently looking to move into a new factory later this year, most probably in Wadeville. So when the turnaround comes, we will also be ready,” he concludes. q

“For us at Axiom, the SAM Hydraulik piston pumps and motors, in various combinations, are at the heart of Axiom hydraulic systems. And there are quite a variety of them. Some are fixed displacement pumps and motors, which produce a directly propor- tional relationship between the rate of flow of oil pumped and the number of revolutions per minute (rpm) of the motor. And they are specified to reflect this. A 125 cc pump unit, for example, displaces 125 cc of oil per revolution, so pumping oil at 125 cc per second gives a rotational speed of 60 rpm. On a fixed displacement pump, while flow control valves can used to vary the speed of hydraulic motors by ‘dumping’ some fluid to the sump through a relief valve – an inefficient process that gener- ates heat – the speed of a fixed displace- ment pump and motor combination can generally only be varied by changing the rotational speed of the diesel engine or electric motor driving the pump. So, where variable speeds are required, fixed displacement pumps are generally unsuitable,” Simpson advises. Variable displacement units, however, with closed loop speed control, are ideal for applications where variable speed control is required. “On a closed loop sys- tem, discharge oil from the motor is fed

drives controlling the bucket of a tower crane allow the

The SAM SH7V series is a family

of variable displacement motors with a bent axis piston design for operation in both open and closed hydraulic circuits. The design incorporates the lens shape valve plate and motors can provide up to 430 bar continu- ous and 480 bar peak performance. operator the very fine control needed to accurately and safely position loads on a construction site,” Simpson relates. While Axiom has standardised on the use of SAM Hydraulic pumps, “we combine these pumps with Black Bruin motors for heavy duty slow speed ap- plications or with SAM Hydraulik mo-

Mechanical Technology — April 2015

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