Connected to a multitude of instruments and sensors, a Sanderson motor undergoes testing at MSOE's Fluid Power Institute. Director Tom Wanke says the Sanderson motor exhibits impressive low-speed performance.
As a function of speed, the SRAM motor torque showed excellent classical behavior, rising sharply from zero, rounding off, and then dropping gradually as the speed increases. The behavior tracks closely that of a radial piston motor, which is known for its excellent low speed torque efficiency.
The remarkable feature of this motor is its performance at reduced displacement. Full and fractional displacements are all within a few percentage points of 90% — unprecedented in the world of hydraulic motors.
Another benefit of the reduced friction is the great reduction in internal heat generated by the SRAM motor. Even at full power, a 36-hp motor rises in temperature only 20° F or less. This may be low enough to eliminate the need for oil coolers in many current applications.
How to start up?
The high partial load efficiency leads to an interesting question. What is the best way to start up a vehicle powered by the SRAM motor? Usually, the operator starts with full displacement, and then increases the pressure until the vehicle starts. With slip-stick, it is usually necessary to spike the pressure to get over the region of unstable operation and into the region of smooth operation.
In the specs for operation, it states that such spiking operations can only occur for one second per minute without, presumably, the danger of overheating. This is apparently the best way to start vehicles because the part load efficiency of all existing hydraulic motors is abysmal.
With the SRAM motor's high part load efficiency, it makes sense to start the vehicle by putting the pressure at its maximum value, and then increasing the displacement gradually from zero. The operation should be smooth without any grabbing, and as the power builds the operator will have complete, manageable control over the vehicle.
Motor design
The present motor design is a prototype of a SRAM hydraulic motor, and there are many potential improvements that Sanderson Engine Development intends to implement. The present SRAM motor was designed with little regard for overall weight, cost of parts, etc. — strictly as a proof of concept machine. The next prototype will address some of these practical design considerations.
Some that can be addressed by simply modifying this prototype are as follows:
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Increased horsepower in same weight. The present design is 36 hp at 3000 psi and 800 rpm and weighs approximately 300 lb. Since the nominal safety factor was about 4:1, the weight to power can be decreased by several easy measures such as testing to higher pressure, hollowing out the now-solid pistons to allow testing to higher speed, boring out the cylinders to accept pistons of about 1.5 times the present diameter, and designing hydrostatically lubricated bearings in the U-joint, in the zero-side-load piston joints, and in the large sleeve joint. These measures will decrease the weight-to-power ratio from 8.3 lb/hp to about 3 lb/hp.
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Cost of parts/hp is reduced by all of the above measures in roughly the same proportion, 8.3 to 3, since only minor expenses were added. In addition to the above changes, SED has been able to eliminate a large roller bearing and replace it with a hydrostatic sleeve bearing at a saving of some $750 in parts. Other savings will undoubtedly occur as the design is reviewed by production engineers, and the size of the production run being analyzed increases from one into the thousands.
Conclusions
This report on the SRAM hydraulic motor will be followed by a report on the advances in SRAM hydraulic pumps, which were first reported in the August, 2005 issue of H&P. The curves will be very similar to the above because the low friction of the SRAM makes the two mechanisms near reciprocals of each other.
