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)

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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)

Hydraulic Hose Burst Valves

Hydraulic Hose Burst Valves
I have talked many times about my dislike for Hose Burst valves (velocity fuse).
A cheap and nasty little valve that screws directly into the cylinder port and should close when the velocity across the valve exceeds the spring setting.
So why do I hate this valve so much?
Many unwitting users do not realise that these valves must be properly adjusted using feeler gauges if they have any chance of them working properly. The manufacturer should supply a chart with the valve showing a valve gap dimension based on flow. This is not the flow of the pump but the flow that will pass from the cylinder under normal conditions and is calculated using the displacement volume of the ram and its velocity.
The valve will not compensate for any change in temperature or load and can often close when operating under normal conditions, causing annoying stoppages. However the main purpose is that they must close should a pipe or hose burst. If the worst happens and the valve does not close, the result can be catastrophic and cause serious damage or injury.
If you are serious about protecting against hose burst then spend a little more money and do the job properly using a Pilot Operated Check valve. It will react instantly and must be built in as part of the cylinder with no pipe between the cylinder port and valve except for the pilot. This is a better quality valve and much more reliable.
Always keep in mind that there is NO complete failsafe way of hydraulically holding up any load with a ram except by using mechanical locking. Even if a new installation initially holds well, time and wear will eventually increase leakage and the load will creep down.
Be safe.
Bob Jackson (The Hydraulicman)
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Oil Viscosity

Viscosity of Hydraulic oil
Often in hydraulic systems the type of oil can be the last thing to consider in the overall design.
ISO 32 mineral oil is the most common Hydraulic oil and I often advise my clients to use this grade of oil because of its availability and compatibility with most pumps, valves and hydraulic components.
The viscosity can be extremely important and I will give a few examples of situations that I have encountered where the oil has been a key factor to resolving problems.
1) We designed a system for an animal park. Large sliding hydraulic doors enclosed Rhinoceros and it was our job to ensure that the doors closed quickly but with minimum force so not to injure the animal. Everything worked to plan but our client complained that on some occasions the electric motor would cut out. We discovered that the motor only cut out on cold days when the oil was at its thickest. The pack worked off 240v 30amp supply and we had no way of increasing the supply. The simple solution was to change the oil from iso 46 to 32. Our customer never had another failure and was very pleased with the simple remedy.
2) An aluminium casting machine uses hydraulic rams to tilt the machine and allow molten metal to fill the mould. When solidified the machine is tilted back to open the die and remove the casting. We had already increased the speed by using a Regen circuit but our customer wanted even more speed. We noticed that there was a difference in speed from morning to afternoon when things began to slow down. By monitoring the temperature we knew this was due to the change in oil viscosity. We recommended he replace the iSO32 for 46 and saved 8 seconds in the total cycle time.
3) In a saw mills proportional valves were used to set adjustable fence. The customer complained that the hydraulics needed to run for at least one hour before the system cut accurately. We changed the oil to thinner grade and immediately improved the accuracy without the need to waste time to bring the oil up to temperature.
The viscosity of hydraulic oil can be very important and often a great problem solver without having to modify expensive hardware.
Always best to check with pump manufacturer to be sure that the pump or motor will not be damaged by a change in oil viscosity.
A great problem solver to keep up your sleeve!
Very best
Bob Jackson
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My Top 7 Hydraulic Faults

I am often called out to Hydraulic breakdowns that take less than 1 minute to find and fix.
This can be an embarrassment for my customer and gives me no pleasure in having to give him a substantial bill for just 60 seconds of my time.
I have listed my 7 top simple faults that could save you a small fortune as well as the costly down time of your piece of machinery.
Check out my top 7 favourite faults before you start looking for more difficult reasons for your Hydraulic problem.
Register on my site www.hydraulicbrain.com for my 7 top faults.
Very best
Bob

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.
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Very best
Bob

The Counterbalance valve

The counterbalance valve is used for load holding and prevents the load running away uncontrolled. The valve automatically throttles the outgoing flow using a pilot signal from the input flow leg to the ram or motor. The valve symbol is very similar to the Relief valve with a normally closed function and a check valve looped around the main valve.
To open the valve it will need a pilot pressure and for very large loads this should be as small as possible where lighter loads should be higher. The valve will be made with a designed pilot ratio, this is ratio of the pilot pressure against the induced load pressure to open the valve and can be typically 3:1 to 10:1
To establish the way to set up the Counterbalance valve you must know the Pilot Ratio. Pilot Pressure= Valve Setting Pressure – Pressure induced by load divided by the Pressure Ratio + the ratio of piston area divided by the annular ram . A hydraulic motor would be 1.
If we are using a hydraulic motor on a winch it will be important to ensure that there is enough pressure to release the brake. This might require a low Pressure Ratio to achieve enough pressure to release the brake.
If you do not set these valves up correctly the system can become unstable, the ram or motor may not move and in the case of winches the brake might not release.
For more about load holding become a member of www.hydraulicbrain.com
Very best wishes
Bob

What is maximum pressure?

These days we expect hydraulic pumps should be fully supported with detailed technical information ensuring that we use the pump in the correct application.
Often we have pages of stuff showing everything from displacement to recommended oil type.
I often find that the pressure duty is not well defined like what pressure can I safely generate without the pump falling apart. You will read descriptions like continuous, intermittent and peak.
Continuous speaks for itself but what about intermittent and peak, what does this description really mean.
Many years ago when I sold hydraulic components for a well known manufacturer the pumps had different ratings for industrial than mobile. For what I can only imagine were commercial reasons, the mobile pump pressures were much higher.
I have also used some pumps well outside their recommended pressure ranges (only for specific applications) and although not a practice I recommend I do wonder what technical basis manufacturers use to determine their pump pressure ratings.
I would like to see a graph with a direct correlation between pressure and life. I want to know that if I design a system that runs once a month for 15minutes at higher than normal pressure should I need to spend much more money on a more expensive pump.
Surely we should be able to get a better idea of pump life without always having to buy the highest spec pump or face the possibility of a bucket of scrap metal after just a few cycles.

Very best
Bob

The Benefits of Hydraulics

With rapid developments in Electric motor drives and the reduction in costs you might wonder if hydraulics is relegated to past technology.
There can be no doubt that electric drives have replaced some hydraulic applications, especially in the machine tool industry. Electric drives are cleaner, more efficient provide better control but hydraulics can still provide a better and often the only solution for many applications. Hydraulics provides better power to weight ratio. You can get a lot of power out of a very small package and where size and weight is important it will be the best choice. For small forces, the electric motor and worm drive can be a “no brainer” but if large forces are required for applications like Earth moving equipment, presses, jacks etc Hydraulics (at the moment) have no better alternatives.
Hydraulics provides other benefits that include:
1) Easy direction control
2) Easy speed control
3) Easy force control
4) Can take shock
5) Linear and rotary drives from the same system
6) No lubrication required
7) You can stall the drive without damage
8) Easy to keep cool
If you specialise in Hydraulic, pneumatic or electrical drive systems always try to use the best solution for the application and not just because you have a personal bias to a particular technology.
Very best Bob
www.hydraulicbrain.com

Dangerous Cranes

I think most will agree that hydraulic crane safety should be of paramount importance and I would like to mention just a few observations about the use of this potentially dangerous piece of machinery.
In my opinion the safest type of crane has been designed to prevent overloading. Not just on the main lift but more importantly where you pick up a load and then telescope out. As the telescope extends increasing its length the main lift cylinder pressure also increases in proportion.
Now some cranes have load sensing devices that will disengage the telescope so that overloading cannot occur and stop the telescope extending when the maximum load is exceeded.
Also the counterbalance valve can also relieve and lower the load in a controlled way.
However there are some cranes that I have come across where they have been fitted with a simple hand valve with all blocked centre ports. These cranes have often been salvaged and the hydraulics replaced by people not proficient in hydraulic systems or crane design. Unwittingly the users of these cranes are just waiting for disaster to strike, at best causing costly damage and at worst serious injury.

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very best
Bob

Problem at Sawmills

Whenever my home phone rings in the early hours of the morning it usually means one thing, trouble!
It was a call from the Sawmills. There pump and servo valve had gone down.
The system drives a carriage complete with log attached, back and forth through a huge band saw. Without the pump and servo the plant comes to a standstill.
The piston pump with servo valve is part of a closed circuit and coupled to a fixed displacement piston motor.
My customer had already decided to replace the hydraulic drive with an electric motor and inverter but delivery one still some weeks away. So was there anything we could do meantime?
Repair was out of the question, the pump was obsolete and repair would take weeks.
I decided to use an 11kw power-pack in storage and couple the motor open circuit. We installed a simple hand valve for the operator and by lunchtime the next day they were cutting wood.
I hope that if they have future problems with the inverter drive they will be able to get the same response?
Very Best
Bob
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Terminally sick systems

Over the years I have been called out to resolve a variety of hydraulic problems but there are some I call the terminally sick system or TSS for short. These are the systems that have gone unnoticed and uncared for that suddenly become mad, panic important because after the many years of abuse and unnoticed service they have finally decided to stop working.
The outside condition will often give the first clue to a TSS. It will be covered in a mixture of blackened oily deposits that has solidified over the years resembling greasy mud. When you look into the tank you recoil from the rancid smell of burnt oil and the colour has no resemblance to the once golden honey colour that was poured in from a new can. On taking a small sample of this gloop you hold it to the light and see a myriad of sparkling metallic particles and now you are convinced that you are dealing with a terminally sick system.
The customer is in panic because he cannot operate his machine and he will have to send everyone home if he can’t get it running within 24 hours and he has called you out because you are the expert and wants it fixed within the next few hours.
Yes, I have made up this story but it is not untypical of the type of breakdowns we are sometimes called out to resolve.
I have my own way of dealing with the TSS but I am more interested in how you would deal with this hydraulic emergency.
Very best
Bob
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Have hydraulic manufacturers become too arrogant?

I am sure that most of you will agree that getting new business and maintaining the existing is something that requires continued hard work. Most of the smaller businesses pay particular attention to giving good services and value their customers. I wish however I could say the same for the large manufacturers of hydraulic equipment. I am not sure if it would be a good idea to name and shame these companies but I guess that you might have a good idea of the ones I refer to.
I have some customers who buy branded products from me rather than deal direct. Not because I can save them money, in fact it’s likely it cost them more. It’s because they do not like dealing with the arrogant attitude so often associated with buying the parts they want. Comments like “You want it now, you must be joking, well if you don’t like 21 weeks delivery you can always go somewhere else!”
I notice that some of these companies do not even want to speak to us anymore. They hide behind a website with no means of contact other than an email address that they never reply to and finding a telephone number can take endless searching. I have been told by distributers that they also find it very difficult.
I rang a top brand servo manufacture for the name of someone who I could contact; I was told that the companies' policy was not to give out any names and that if I wanted I could send an email. The next time their rep knocks on my door or rings me up for an appointment you can guess what I will say to them!
We spend a great deal of time, effort and money trying to create a great experience for our customers as I am sure you guys do. We want our businesses to grow and be successful but some of these big companies have lost their desire or ability to give good service. Unfortunately for us we often have no choice but to continue to buy from them.
If you have had any experiences like me or disagree with my analysis I would love to hear from you.
Cheers
Bob

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Grooves in Pistons

I was recommended a book “Hydraulic Control Systems” by Herbert E Merritt. Although the maths flies gently over my head there is some very interesting stuff for the Hydraulic enthusiast.
I recently asked a question about grooves in pump pistons and valve spools and what was their purpose. I found the answer and I will try to explain in my own Layman’s language.
With close tolerance pistons it is possible for hydraulic forces to create lateral movement on the piston forcing it to one side of the bore. This can cause stiction (hydraulic lock), extra leakage and a trap for contamination.
Just one groove on the piston can reduce the lateral forces by up to 40%. The grooves create a centreing effect and help prevent metal to metal contact between piston and bore. Adding more grooves will also be beneficial.
The depth and the width of the groove should be 10 times the clearance and the sides of the groove should be perpendicular to the bore.
Ref from Hydraulic Control Systems

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Things you remember

Anyone with a long career in hydraulics, I am sure will remember those times when things went badly wrong and ended with a few sleepless nights. The one etched in my distant memory was an application for a hydraulic riveting system. The client need several pressures all quite set close to each other. On the face of it a very simple straightforward project. We tested the system in the workshop and all seemed well before delivering it to the customers factory.
Within a few days we had the first of many phone calls.
It seemed that without warning the system started screaming so loud that the workers had to cover their ears.
By the end of the week the power pack was back. We tried all sorts of things to resolve the problem and each thing we tried we thought it was fixed, only to discover that within a few days it would start playing up again with the ear shattering noise.
In the end we admitted defeat and re reimbursed the client in full.
For me, at the beginning of my hydraulic career it was a huge blow and I considered that maybe I was not cut out for a future in hydraulics.
But you eventually move on and get bad experiences behind you with more successful jobs. However when I come across an application with two Relief valves in the same circuit I feel that familiar sickness again envelope my being.
Regards
Bob

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What is Regen?

Regenerative (Regen):
This often causes a little confusion with newcomers to hydraulics and I hope that I can make it easy to understand. We use the Regen circuit when we need a fast extend e.g approach cycle on a machine tool. Very simply, when extending we only displace the rod volume not the cylinder volume.
This means that the rod will move out faster but will have a much reduced force due to its area.
When we reach the point where we need the large force then we can switch out the Regen and apply pressure over the whole piston area. The ram will now move slower but we only need force not speed. Regen can only work on the extend stroke, however the ram will return quickly due to the anular area. 
Be careful! with too large a rod the benefits may not be worth while. If rods are too small the force may not be large enough to overcome seal friction.      
Bob
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