What is in this article?:
- Designing for shock and violence
- Dealing with pressure spikes
Chewing up cars, appliances, tires, and other tough waste into fragments requires powerful drives that stand up to heavy shock and instant reversals.
The ultimate goal for a shredding system is to reduce the source material — appliances, tires, concrete, etc. — to smaller particles for recycling. To transmit the low-speed, high-torque power needed for these heavy-duty shredders, a gearbox could be used to reduce speed from an electric motor while multiplying torque. However, hydraulics technology is a more practical solution both for systems driven through gearboxes and those that utilize low-speed, high-torque motors in direct-drive systems.
To ensure reliable delivery of hydraulic power, pumps are generally used for the drive application for the cutter shafts where a series of rotating cutters pulls debris from a hopper through small openings to shear and reduce its bulk. These pumps have a maximum continuous operating pressure of 5000 psi (344 bar) and a maximum theoretical flow of 238 gpm at 1800 rpm. In typical radial-piston designs, the pump may feed two pairs of hydraulic motors, with each mounted to either end of the cutter shafts.
When it comes to the hydrostatic transmission design, shaft speed and operating torque are only the beginning. In reality, shredding produces a considerable amount of shock in the system. Envision driving your car at 100 mph, slamming on the brakes, then immediately going 100 mph in reverse. That’s what happens in shredding when the cutters jam — they reverse direction instantly, an extremely violent event! Furthermore, because the cutters can shear in both directions, it can take from one to ten revolutions of the cutter shaft to create a “violent event” prior to the reversal cycle.
Controlling when the cutter shafts reverse — and how long the reverse or forward cycle lasts — is programmable on some machines (within limits designed into the program). These programmable values can help to evenly distribute the loads, potentially extending shredder and hydrostatic transmission life, as well as the time between maintenance cycles.
Violent events can cause instant pressure spikes from 700 to 5000 psi (48 to 344 bar) within 100 msec. At the same time, the hydraulic hoses going to the pump from the motor can swell up, acting like a hydraulic accumulator. This swelling action can be detrimental if the replenishing system cannot make up for the instant loss of oil — as much as 100 gpm for the 100 msec. The total volume lost is rather small, but the instantaneous loss is substantial.
Because of their quick response, bladder type accumulators often are added to the system to aid replenishing, depending on hose length. You must take this accumulator effect into account for all types of hydrostatic transmissions during the initial design phase. Shorter hose lengths are best, but not always possible. A 1-gal accumulator is typical in applications with long hose runs. To accurately determine flow loss due to expansion from pressure, check with the hose manufacturer for bulk modulus values for different types of hose.
|Photo shows the Gold Cup valve block with arrows indicating hose attachment ports.|
Because of the repetitive violent pressure spikes, the type of fluid used is also important. Whether petroleum base, or synthetic, fluids with a ZDDP (zinc dialkyldithiophosphate) anti-wear package are far superior to those with friction modifiers. Plus, it is good practice to check the fluid on a set schedule in conjunction with independent lab analysis to plot trends in contamination amount, type, and water content. Make sure the fluid maintains the additive package and viscosity. Pump and motor manufacturers can recommend parameters for maximum efficiency and performance.