All ports blocked control valve

Beware The All ports blocked Control valve
All hydraulic systems leak to some degree and any suspended load held on a ram will eventually creep down due to gravity. However there are occasions when a ram will creep out when the ram is mounted horizontally or even pointing upwards.
Whenever I am asked to investigate this fault I always look for the following:
1) Pressure is applied to the valve all the time (usually pressure compensated pump).
2) The control valve has a closed centre position (all ports blocked).
If we analyse this arrangement we must consider what is happening to the control valve. Pressure is being applied to the closed port (P). The A&B ports are situated either side of the pressure port (P) and the tank ports (T) are furthest away. When the valve is brand new the leakage is minimal but as time passes spool wear increases together with leakage from port P (pressure) to A&B. The same pressure applied to both sides of a ram will cause it to extend due to the difference In the area (full bore and annular).

There are two ways you can resolve this problem.
1) Replace the control valve with same.
2) Replace the valve with centre position P blocked with A&B to T and add a double PO check valve.
In this set up any leakage across P will flow directly to tank and should not create enough pressure to force the ram out. The PO check valves will lock the ram in position against external loads.
Hope you find this helpful
Bob Jackson (The Hydraulicman)
More of this can be found at www.hydraulicbrain.com

Hydraulic design is like a day's fishing

When I was a small boy, nothing was more exciting than going for a day’s fishing. Not that I can remember catching much but it was the anticipation that it might just be the day when I would catch a whopper!
Many years later I now see how the experts do it and it is hardly surprising I caught very little. As a child I used the same method of fishing regardless of the conditions, location or the type of quarry I hoped to land.  Top fishermen plan their task and select the right equipment to suit the conditions.
If they get it wrong, at best they will catch very little and at worse loose or break their equipment because the fish was too large for the tackle they selected for the task.
Now let us consider the Hydraulic designer, he must also consider the task. What are the loads and speeds required?  Where will the equipment be used, (hot, cold, wet or dirty) and from a basket of pumps, valves, actuators and controls, select the right parts for the best result. Get the selection wrong, at best will not achieve the task and at worse cause significant damage to your customers equipment. Get it right and it’s as satisfying as catching the Whopper!
Good fishing.
Bob
More Hydraulics at www.hydraulicbrain.com                  

Hydraulic pump suction strainers

I return to an old subject, should we fit pump suction strainers?
I posed this question to my Linked in forum (Hydraulic help) and the overwhelming opinion was to leave them off.
For those just starting out in hydraulics let me briefly recap on the purpose of suction strainers.
As its name implies this is not a typical filter but simply a piece of 125micron metal gauze rolled into a tube that fits to the end of the pump suction pipe. Particles over 125 micron will be blocked thus preventing them entering the pump possibly causing instant destruction. This sounds very admirable and because of their low cost one would see no reason for not installing them, however there is a downside.If the suction line is restricted Hydraulic Pumps will quickly deteriate due to cavitation. 
Suction strainers can be difficult to change and most often end users are totally unaware that they even exist. Over time this will result in the gauze silting up and blocking the oil flow to the pump and may even collapse the strainer with catastrophic results.
Many engineers say that if the tank is very clean and free of all weld splatter, the strainer will serve little purpose.  This strategy however is not risk free because you must be 100% sure that there is no way that any contaminant can drop into the tank, that the paint inside will not flake or peel and that hoses and pipework have been thoroughly cleaned prior to start up.
My opinion is, look at the application and risk assess the environment, housekeeping abilities and responsibilities of the user and the implications of serious pump failure with the consequence of serious contamination throughout the system and potential downtime to the plant. Also talk to your customer about these issues.
keep safe
Bob Jackson
More hydraulics on www.hydraulicbrain.com