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
Saturday 31 March 2012
Tuesday 20 March 2012
What is the difference between airation and cavitation?
Often miss diagnosed and not surprising because the symptoms are similar but with very different causes.
Airation
Very simply this is the result of air getting into the hydraulic fluid and most likely through the suction pipe or if the oil level has been allowed to get too low, partially uncovering the inlet to the pump. The pump will sound noisy (lumpy), a very similar noise to cavitation and undesirable for the following reasons.
1) The system becomes spongy and movements become erratic with pressure fluctuations.
2) The lubrication of the sliding parts within the pump etc will be reduced and lead to failure.
3) Dieseling effect could cause ignition with very high temperature spots, damaging seals
The Cure: The suction pipe will be subject to vacuum so do not expect to see any oil leak so best check all fittings between the strainer and pump inlet. Another way is a little messy but try grease or oil around the joints and listen for pump tone change.
Cavitation There are many reasons for cavitation but they all result in restricting oil entering the pump.
Best to start with easy stuff first:
1) Tank breather blocked (remove and replace)
2) Suction valve not fully open
3) Oil cold or too thick (check viscosity)
4) Suction Strainer clogged (remove and replace)
5) Suction pipe too small
6) Suction pipe too long
7) Pump speed too high
Cavitation is very destructive and will damage the pump in a very short time. Minute explosions take place on the surface of the metal during the transition from vacuum to high pressure with temperatures reaching greater than that on the sun. Pitting and metal removal will quickly lead to failure.
Hope you find this interesting and helpful and you can find more valuable information if you sign up at www.hydraulicbrain.com
Keep safe
Bob Jackson (The Hydraulic man)
Airation
Very simply this is the result of air getting into the hydraulic fluid and most likely through the suction pipe or if the oil level has been allowed to get too low, partially uncovering the inlet to the pump. The pump will sound noisy (lumpy), a very similar noise to cavitation and undesirable for the following reasons.
1) The system becomes spongy and movements become erratic with pressure fluctuations.
2) The lubrication of the sliding parts within the pump etc will be reduced and lead to failure.
3) Dieseling effect could cause ignition with very high temperature spots, damaging seals
The Cure: The suction pipe will be subject to vacuum so do not expect to see any oil leak so best check all fittings between the strainer and pump inlet. Another way is a little messy but try grease or oil around the joints and listen for pump tone change.
Cavitation There are many reasons for cavitation but they all result in restricting oil entering the pump.
Best to start with easy stuff first:
1) Tank breather blocked (remove and replace)
2) Suction valve not fully open
3) Oil cold or too thick (check viscosity)
4) Suction Strainer clogged (remove and replace)
5) Suction pipe too small
6) Suction pipe too long
7) Pump speed too high
Cavitation is very destructive and will damage the pump in a very short time. Minute explosions take place on the surface of the metal during the transition from vacuum to high pressure with temperatures reaching greater than that on the sun. Pitting and metal removal will quickly lead to failure.
Hope you find this interesting and helpful and you can find more valuable information if you sign up at www.hydraulicbrain.com
Keep safe
Bob Jackson (The Hydraulic man)
Thursday 15 March 2012
What is pressure compensation?
So what is pressure compensation?
For any flow of fluid through a pipe there must be a pressure drop with greater pressure at the source of flow (pump) and lessoning the further you go downstream. This is due to the frictional losses in pipe, fittings, valves, filters etc and we call this Pressure Drop or Delta P and often signified using the symbol (ΔP). If there is no difference in pressure then there will be no flow.
If the pressure difference (ΔP) is constant then the flow and hence the speed of the actuator will also be constant but if the flow or load changes there will also be a change in the pressure drop and consequently change in speed of the actuator.
With some applications, constant speed of a ram or motor can be very important, take an application like a hydraulic passenger lift. It is important that the speed with only one passenger is the same as when the lift is full of people. Without compensating for the pressure difference the speed would change. With more people in the lift the downward speed would increase and with just one person might be unacceptably slow.
So we need a valve that will automatically close down and reduce flow when the load is greater and open when the load is less or a pump that will increase flow or decrease flow to maintain a constant Delta P.
With a speed control valve make sure it is a pressure compensated type and sized correctly for the flow. To maintain Delta P with the pump use a load sensing option. Variable piston and vane pumps only have this option available. A load sensing pump will automatically adjust the flow to the required load and maintain the correct speed. Another benefit with this type of arrangement is power saving and less heat generation.
Bob Jackson (The Hydraulic man)
More interesting stuff at www.hydraulicbrain.com
For any flow of fluid through a pipe there must be a pressure drop with greater pressure at the source of flow (pump) and lessoning the further you go downstream. This is due to the frictional losses in pipe, fittings, valves, filters etc and we call this Pressure Drop or Delta P and often signified using the symbol (ΔP). If there is no difference in pressure then there will be no flow.
If the pressure difference (ΔP) is constant then the flow and hence the speed of the actuator will also be constant but if the flow or load changes there will also be a change in the pressure drop and consequently change in speed of the actuator.
With some applications, constant speed of a ram or motor can be very important, take an application like a hydraulic passenger lift. It is important that the speed with only one passenger is the same as when the lift is full of people. Without compensating for the pressure difference the speed would change. With more people in the lift the downward speed would increase and with just one person might be unacceptably slow.
So we need a valve that will automatically close down and reduce flow when the load is greater and open when the load is less or a pump that will increase flow or decrease flow to maintain a constant Delta P.
With a speed control valve make sure it is a pressure compensated type and sized correctly for the flow. To maintain Delta P with the pump use a load sensing option. Variable piston and vane pumps only have this option available. A load sensing pump will automatically adjust the flow to the required load and maintain the correct speed. Another benefit with this type of arrangement is power saving and less heat generation.
Bob Jackson (The Hydraulic man)
More interesting stuff at www.hydraulicbrain.com
Saturday 3 March 2012
Eco Friendly Hydraulic Oil
The oil that flows through hydraulic systems can almost be compared with blood which flows through our bodies and like blood; any problems with the oil can affect components, performance or serious failure of the hydraulic system. Hydraulic oils can vary in their formulations using different additives to improve the system performance and protect components against wear and corrosion. In recent years new synthetic oils have become available and the ones most interesting to me are those that cause less damage to the environment if accidentally spilt on land, sea or rivers.
When we designed the recent hydraulic system for the open 60 racing Yacht “Acciona” our client stressed that the hydraulic system should present no risk of contamination to the sea. The amount of oil carried on this type of boat is very small compared to industrial applications but even one litre of mineral oil accidentally spilt into the sea is still far too much and not acceptable if we are serious about keeping our oceans free of pollution.
Rape seed oil has been available for many years for environmentally sensitive applications but do not offer the same protection to hydraulic components as mineral oils but synthetic based oils are a new generation of “Eco” oils that offer greater benefits for systems and environment.
We contacted Millers Oils who recommended Millmax 32HFD(u).
I wanted to know (in laymen terms) how effective the oil was in protecting the environment and hydraulic components and this is their reply:
Definitely would not recommend you drink it, would have a similar effect to drinking Castor Oil.
Would not kill fish as any escaped oil would lie on the surface oil while it was biodegrading away (being eaten by bacteria)
Synthetic esters which is made from are reaction products of fatty acids and alcohols, careful selection of the two components can radically change the end product.
They will provide better protection than mineral oils and also give better lubricity (more slippery) providing the oil stays at sensible temperature i.e. kept below 80˚C they will outlast a mineral oil
For us it is still early days to properly evaluate this oil but so far it has performed well. At the moment these oils can be five times the price of mineral oil but if it is as good as the manufacturer’s claim then I believe that we should be consider it for more applications especially where there is any risk to the environment.
Bob Jackson (The Hydraulicman)
For more hydraulics see www.hydraulicbrain.com
When we designed the recent hydraulic system for the open 60 racing Yacht “Acciona” our client stressed that the hydraulic system should present no risk of contamination to the sea. The amount of oil carried on this type of boat is very small compared to industrial applications but even one litre of mineral oil accidentally spilt into the sea is still far too much and not acceptable if we are serious about keeping our oceans free of pollution.
Rape seed oil has been available for many years for environmentally sensitive applications but do not offer the same protection to hydraulic components as mineral oils but synthetic based oils are a new generation of “Eco” oils that offer greater benefits for systems and environment.
We contacted Millers Oils who recommended Millmax 32HFD(u).
I wanted to know (in laymen terms) how effective the oil was in protecting the environment and hydraulic components and this is their reply:
Definitely would not recommend you drink it, would have a similar effect to drinking Castor Oil.
Would not kill fish as any escaped oil would lie on the surface oil while it was biodegrading away (being eaten by bacteria)
Synthetic esters which is made from are reaction products of fatty acids and alcohols, careful selection of the two components can radically change the end product.
They will provide better protection than mineral oils and also give better lubricity (more slippery) providing the oil stays at sensible temperature i.e. kept below 80˚C they will outlast a mineral oil
For us it is still early days to properly evaluate this oil but so far it has performed well. At the moment these oils can be five times the price of mineral oil but if it is as good as the manufacturer’s claim then I believe that we should be consider it for more applications especially where there is any risk to the environment.
Bob Jackson (The Hydraulicman)
For more hydraulics see www.hydraulicbrain.com
Saturday 25 February 2012
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
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
Sunday 19 February 2012
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
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
Saturday 11 February 2012
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
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
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