One of our members wrote me this question:
“In hydraulic systems and in hydraulic components in particular, bolts are everywhere. Should we change bolts every time we open them, for example, on a hydraulic cylinder head joint? If not, how many times can bolts be re-torqued?”
Unlike bolts used on engine heads, most of the bolts used to assemble hydraulic components and port flanges are not ‘torqued to yield’. In other words, bolts used on engine heads are torqued beyond their elastic limit and therefore using them a second time is fraught with danger. This is generally not the case … continue reading »
In chapter 13 of The Hydraulic Maintenance Handbook, I discuss the principle of engineering out issues that have the potential to cause recurring maintenance problems. Since maintenance occurs after the event, that is, after the machine is designed and built, this strategy is very much a retrospective one. Which means it’s desirable and preferable not to engineer in such potential problems in the first place.
And I was reminded of this recently. While conducting failure analysis for a client, small flakes of paint were found to be present on and in parts of the failed piston pump’s rotating group. … continue reading »
One of the key advantages of direct-acting relief valves, in comparison to their pilot-operated cousins, is fast response. And their fast response makes direct-acting reliefs the valve of choice in situations where pressure spikes are possible and must be minimized. Secondary relief valves in an actuator circuit and the ‘spike relief’ for a variable-displacement pump are common examples.
But as a general rule, direct-acting relief valves suffer more from pressure override than their pilot-operated counterparts. Pressure override is the difference between the valve’s full flow pressure and its cracking pressure.
For example: if a relief valve is set so that … continue reading »