Know your RPM

I was asked to oversee the installation of a hydraulic winch system installed on a workboat and designed by (winch experts)
My client had used this company before and was very satisfied with their expertise and service. If my client was happy then so was I.
It came the day when I had to inspect the installation. There was a gear pump attached to a large diesel engine via a flexible coupling, the winch was fitted on deck and the control valve, counterbalance valve with manifold were supplied loose.
I studied the circuit and the first thing that caught my attention was the closed centre control valve. This prompted an immediate phone call to the designer, the last thing we wanted was a hot system. He assured me that the control valve supplied was in fact a Pressure to Tank configuration and that he had drawn the symbol incorrectly. I then asked him what speed had he designed the pump to run at, “1000rpm” he said.
Now under normal conditions whilst using the winch the engine speed would run at 1000rpm but when the boat steamed from one position to another the engine rpm can increase to 2100-2200rpm, more than twice the design speed and what is more important twice the design flow.
Now we have a problem!
The control valve and pipe-work was not large enough to handle this extra flow. The first suggestion by the designer was to increase the size of the control valve, an obvious solution you might think. The first problem was that it would need larger diameter hoses (difficult to get onto the valve) but more importantly the valve looses its sensitivity due to the flow gain of the larger valve. Being a proportional valve and using a joystick with electrical control the winch would not have the sensitivity and less easy to control at low speed.
The better option is to fit and unloading valve directly off the pump outlet. When unloaded half the flow could pass through the unloading valve and half through the control valve. When the engine speed is reduced to 1000rpm the unloading valve can be closed allowing the reduced flow to pass to the control valve and operate the winch at the correct speed.
When driving pumps using engines especially with fixed displacement pumps you should always consider the working and the maximum rpm of the pump. It might be necessary to adjust the specification to cater for the extra flow.
Attributed to: Omar Khayam, 13th century philosopher
He who knows not, and knows not that he knows not, is a fool. Shun him.
He who knows not, and knows that he knows not, is a child. Teach him.
He who knows, and knows not that he knows, is asleep. Wake him.
He who knows, and knows that he knows, is a leader. Follow him.

Very best
Bob Jackson (The Hydraulic man)

More hydraulics at www.hydraulicbrain.com

Hydraulic Safety

Hydraulic Safety
Anyone involved with Hydraulic equipment can be subjected to considerable risk unless good working practice is always adopted. Hydraulic machines are often very powerful with large dynamic forces and high pressure fluid that can also sometimes be scalding hot.
When working on hydraulic equipment it is a good idea to do an assessment of risk (download example from www.hydraulicbrain.com)
Never work on systems until you are sure that the pump is switched off and all suspended loads are lowered so nothing can fall or move should you remove pipe-work.Accumulators are a source of large potential energy so ensure they are completely discharged.
Prevent equipment being started up by third parties. Maybe you can lock the system,remove the key,electricalfuses and put up signs to say that the system is being worked on and must not be used
There are many dangers for the Engineer and many things that you can do to make yourself safe.
For more stuff on Hydraulic Safety log on to www.hydraulicbrain.com
Very best
Bob