June 15, 2004 

'Inside Hydraulics' Newsletter

1. Catastrophic hydraulic cylinder failure
2. Reducing noise emission from hydraulic systems
3. More hydraulic maintenance tips
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Catastrophic hydraulic cylinder failure

One of our readers wrote to me recently regarding the following incident:

 "A hydraulic elevator ram (single acting - rated test pressure 315bar - working pressure 210 bar) was under test at 280bar. The rod was fully extended in the vertical position and would normally be under test like this for about 15 mins. After less than a minute, the cylinder failed catastrophically at the top gland, actually firing the rod out of the cylinder a distance of about 35 feet into the air. No one was hurt, damage was limited to the building. When inspected afterwards, the top of the cylinder appeared to have been 'flared' outwards. What would cause this type of failure?"

The root cause of this failure was inadequate purging of air from the cylinder prior to pressure testing. To understand why, read this article.

2.   Reducing noise emission from hydraulic systems

Many industrialized countries have regulations that restrict noise levels in the workplace. The high power density, and corresponding high noise emission of hydraulic components means that industrial hydraulic systems are often the target of efforts to reduce mean noise levels in the workplace.

The dominant source of noise in hydraulic systems is the pump. The hydraulic pump transmits structure-borne and fluid-borne noise into the system and radiates air-borne noise.

All positive-displacement hydraulic pumps have a specific number of pumping chambers, which operate in a continuous cycle of opening to be filled (inlet), closing to prevent back flow, opening to expel contents (outlet) and closing to prevent back flow.

These separate but superimposed flows result in a pulsating delivery, which causes a corresponding sequence of pressure pulsations. These pulsations create fluid-borne noise, which causes all downstream components to vibrate. The pump also creates structure-borne noise by exciting vibration in any component with which it is mechanically linked, e.g. tank lid. The transfer of fluid and structure-induced vibration to the adjacent air mass results in air-borne noise.

Reducing fluid-borne noise

While fluid-borne noise attributable to pressure pulsation can be minimized through pump design, it cannot be completely eliminated. In large systems or noise-sensitive applications, the propagation of fluid-borne noise can be reduced by the installation of a silencer. The simplest type of silencer used in hydraulic applications is the reflection silencer, which eliminates sound waves by superimposing a second sound wave of the same amplitude and frequency at a 180-degree phase angle to the first.

Reducing structure-borne noise

The propagation of structure-borne noise created by the vibrating mass of the power unit (the pump and its prime mover) can be minimized through the elimination of sound bridges between the power unit and tank, and the power unit and valves. This is normally achieved through the use of flexible connections i.e. rubber mounting blocks and flexible hoses, but in some situations it is necessary to introduce additional mass, the inertia of which reduces the transmission of vibration at bridging points.

Reducing air-borne noise

The magnitude of noise radiation from an object is proportional to its area and inversely proportional to its mass. Reducing an object's surface area or increasing its mass can therefore reduce its noise radiation. For example, constructing the reservoir from thicker plate (increases mass) will reduce its noise radiation.

The magnitude of air-borne noise radiated directly from the pump can be reduced by mounting the pump inside the tank. For full effectiveness, there must be a clearance of 0.5 meters between the pump and the sides of tank, and the mounting arrangement must incorporate decoupling between the power unit and tank to insulate against structure-borne noise. The obvious disadvantage of mounting the pump inside the tank is that it restricts access for maintenance and adjustment.

If hydraulic system noise remains outside the required level after all of the above noise propagation countermeasures have been exhausted, encapsulation or screening must be considered.

"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.   More hydraulic maintenance tips

For more hydraulic maintenance articles like the ones above, read Brendan Casey's regular column in Machinery Lubrication. This magazine contains practical information on a wide range of maintenance issues including lubricant selection and application, contamination control, filtration and condition-monitoring of hydraulic and other rotating machinery. To receive a complimentary subscription to this informative magazine (US and Canada only) go to: http://www.machinerylubrication.com/hydraulic1.asp

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 editor@hydraulicsupermarket.com 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 16 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: http://www.InsiderSecretsToHydraulics.com

5. Help us spread the word

If you've found our 'Inside Hydraulics' newsletter interesting and informative, then chances are you have a colleague who would too. Help spread the word about 'Inside Hydraulics' by forwarding this issue to a colleague. If they share your interest in hydraulics, then they will surely appreciate being told about this newsletter.

New subscribers can get the newsletter by completing the form at http://www.insidersecretstohydraulics.com

6. Tell us what you think

We would love to hear what you think of this issue of our 'Inside Hydraulics' newsletter. And of course, if you have any suggestions for future issues, please send us those too.

Just e-mail the editor at: newslettersuggestions@hydraulicsupermarket.com

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