Bench Degree·FLUID POWERchapter

Before You Start

One page. The parts list for the whole volume is Appendix A, at the back of the book; Chapter 1 will tell you what it needs. This page is the part you should not find out later.

This volume has a hard limit on it, and the limit is where the subject stops being a bench subject.

Nothing in this book asks you to build, modify or repair anything running above about 1,000 kPa (150 psi). Everything on the kit list works at or below that, and it is enough to measure Pascal’s law, area ratios, the compressibility difference, flow against pressure, cylinder force in both directions, orifice behaviour, pressure drop, air consumption, leak rate and the force curve of an artificial muscle.

Above that pressure this book teaches you to read, measure and diagnose, and never to build. Two reasons, both specific.

Skin is penetrated by a fluid jet at somewhere around 7,000 kPa (1,015 psi). Every hydraulic machine in Chapters 16, 18 and 19 runs at three to five times that. A pinhole in a hose at 21,000 kPa (3,045 psi) produces an invisible jet at 220 m/s (720 ft/s) that goes through a hand with almost no pain at the time and destroys the tissue over the following hours. Chapter 22 Section 1 is that argument in full, and it is not a formality.

And a hydraulic hose assembly is not a garage matter. Hose, fittings and crimp dies are a matched system, crimped to a specified diameter with calibrated equipment, and a hose assembled with hose clips or compression fittings is a hand grenade with a delay on it.

Read Chapter 22 before you go near any machine above 1,000 kPa (150 psi). Read Section 5 on injection injury and Section 6 on stored energy. A machine that has been switched off correctly still holds everything an accumulator, an air receiver, a raised load or a sun-warmed hose was holding before, and none of those makes any noise.

Two items of safety kit are not optional and they cost almost nothing: impact-rated safety glasses, worn for every experiment involving pressure including the ones with a garden hose, and a piece of card for finding leaks. Never run a hand along a hose to find a leak. Both are specified in Appendix A.

Bench Degree

Get the degree without the diploma.

Learn the material, not how to pass the exam.