May 1, 2007 

'Inside Hydraulics' Newsletter

1. How to minimize stall and pressure spikes in hydrostatic transmissions
2. A fraction of a second is all this takes ...
3. How to synchronize hydraulic cylinders
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


How to minimize stall and pressure spikes in hydrostatic transmissions

In response to my article in last month's issue on the accumulator effect of conductors combined with oil compressibility and the problems it can cause in a hydrostatic transmission, Fred Paul, Principal Engineer at SPEC in Texas wrote me this message:

"Brendan, I enjoyed your article in Issue #61 and thought I'd share a story of my own. In 1981, I was working for Hughes Tool Company in Western Australia babysitting a giant drill rig that we designed for Santa Fe International. It was being used to drill ventilation shafts at a nickel mine near Kalgoorlie. The ventilation shafts were 5 meters in diameter and about 500 meters deep.

I designed the hydraulic system for the Hughes Combination Shaft Drill (CSD) 300 based upon an older and smaller machine. The CSD 300 used six Series 27 Sundstrand motors each with a displacement of 20.36 (334 cc/rev) mounted on 16:1 planetary gear reducers that drove a massive 84" diameter bull gear. Nominal drilling torque was 500,000 ft-lbs at 3000 PSI with a maximum torque of about 850,000 ft-lbs. One of the lessons we learnt on the earlier machines was how to minimize pressure spikes in the hydrostatic drive.

The minimization of the pressure spikes was achieved simply by using flywheels placed between the hydraulic motors and the planetary drives. The stored energy in the flywheels allowed the drive to maintain speed and torque with a minimum amount of hesitation during the roughest of drilling encounters.

Far too often, I have witnessed a drive that failed to perform because the torque requirements of the hydrostatic system were underestimated for variations in load that occur. Engineers scratch their heads and contemplate a complete redesign when a relatively small flywheel in the drive train would have allowed their system to perform flawlessly."

Many thanks to Fred for sharing this pearl of wisdom with us all.

The Nice Things People Say

"Brendan, I am excited. I just downloaded your and Marian's writings on Variable Pump Control and Electro-Hydraulic Control Systems they are extremely well thought out with easy to understand terminology. Well done and thanks for making them available at a realistic price."

Kevin McCaffrey
Excel Hydraulics
Sydney Australia

2.   A fraction of a second is all this takes ...

Seeing that the issue of over-pressurization has been raised, I thought I'd continue on the same thread. When a hydraulic system sees a spike in pressure it won't necessarily blow up with a bang. But damage can occur in a number of ways. In fact, a single pressure spike of sufficient magnitude can render a piston pump or motor unserviceable. Here's how:

In axial and bent axis piston pump and motor designs, the cylinder barrel is hydrostatically loaded against the valve plate. To maintain full-film lubrication between the rotating cylinder barrel and the stationary valve plate, the hydrostatic force holding them in contact is offset by a hydrostatic force acting to separate the parts. This is achieved by making the effective area of half the total number of piston bores slightly larger than the effective area of the pressure kidney in the valve plate.

The higher the operating pressure, the higher the hydrostatic force holding the cylinder barrel in contact with the valve plate. However, if operating pressure exceeds design limits, the cylinder barrel will separate from the valve plate.

Design geometry prevents a perfect alignment of the opposing hydrostatic forces. This misalignment creates a twisting force (torque) on the cylinder barrel. During normal operation, this torque is supported by the drive shaft (axial designs) or center pin (bent axis designs). If operating pressure exceeds design limits, the magnitude of the torque created causes elastic deformation of the drive shaft or center pin. This allows the cylinder barrel to tilt, bearing hard against the outlet side of the valve plate and separating from the inlet side (exhibit 1).

valve plate separation

Exhibit 1. Separation of cylinder barrel and valve plate
due to overpressurization
(Bosch Rexroth)

Once separation occurs, the lubricating film is lost and the resulting two-body abrasion damages (scores) the sliding surfaces of the cylinder barrel and valve plate. Erosion of the kidney area of the valve plate can also occur as high-pressure fluid escapes into the case at high velocity. This surge of flow into the case can cause excessive case pressure, resulting in shaft seal failure.

Note also that separation can also occur at operating pressures within design limits due to distortion (loss of flatness) of the valve plate, over-speeding or excessive wear of the cylinder barrel drive-spline in axial designs.

Editors Note: If you're reading this in Queensland, it's your last chance to have Brendan Casey show you how to reduce your operating costs by preventing unnecessary hydraulic failures, step-by-step, in a one-day, Hydraulic Breakdown Prevention Workshop he's presenting on the Gold Coast on May 25, 2007. (Other dates around Australia in 2007). So download the details, and register today.

"This book has the potential to save many organizations lots of m0ney. It should be on the bookshelf of every engineer, supervisor, planner and technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more

Alexander (Sandy) Dunn
Plant Maintenance Resource Center

3.   How to synchronize hydraulic cylinders

The problem of synchronizing multiple hydraulic cylinders arises in many applications. This Instant Knowledge report introduces and explains eight different solutions to this problem. To assist you in selecting the most suitable solution for your application, each method is considered according to its level of accuracy, cost and complexity. The methods discussed are applicable to any number and any type of hydraulic actuator. Find out more

4. Content for your web site or e-zine

Need some fresh content for your web site or e-zine? You now have permission to reprint these 'Inside Hydraulics' articles on your web site or in your e-zine, provided:

1. Each article is printed in its full form with no changes.

2. You send an e-mail to to advise us where you'll be publishing them.

3. You include the following acknowledgement at the end of each article:
About the Author: Brendan Casey has more than 17 years experience in the maintenance, repair and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his web site:

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