When a system requires more than one ram or motor the first question to ask is do they work at the same time. If they do you must now consider the implications. Using hand valves might just result in less speed but because oil always takes the least resistance it might stop one movement altogether.
If in doubt I prefer to use one pump for each device and then I am sure that one “mover” will not affect the other. It’s very difficult to put right if you get this wrong!
If you know a better and reliable way I would love to hear from you.
Cheers
Bob
Sunday, 12 June 2011
Monday, 6 June 2011
Pump Design
I am sure pump designers would love to find ways to completely eliminate, Pressure drop across the pump (with free outlet) together with, friction, noise and heat. (wasted energy) but this is the price we pay to get the flow through the pump. These are the “undesirables” of pump design we must live with (due to physics) although with better design and technology we continue to improve.
Flow is the desirable product we want from the pump and it is important that we make the pump strong enough to resist any restriction of that flow up to a maximum design pressure. After that we will likely cause damage to the pump.
So how does the pressure change within the pump?
1) By increasing pump speed (increasing the flow)
2) Changing the oil (resisting flow)
3) Adding a load (resisting flow)
4) Pipe size (resisting the flow)
And so on..
With a load sensing pump we sense a change of pressure (due to change in load) across the control valve and the pump will automatically respond and make corrections to maintain Delta P. This pressure adjustment can only be achieved by adjusting the pump flow. .
This is why we say pumps produce flow not pressure but they are designed to take pressure
Flow is the desirable product we want from the pump and it is important that we make the pump strong enough to resist any restriction of that flow up to a maximum design pressure. After that we will likely cause damage to the pump.
So how does the pressure change within the pump?
1) By increasing pump speed (increasing the flow)
2) Changing the oil (resisting flow)
3) Adding a load (resisting flow)
4) Pipe size (resisting the flow)
And so on..
With a load sensing pump we sense a change of pressure (due to change in load) across the control valve and the pump will automatically respond and make corrections to maintain Delta P. This pressure adjustment can only be achieved by adjusting the pump flow. .
This is why we say pumps produce flow not pressure but they are designed to take pressure
Thursday, 2 June 2011
How would you define a pump?
How would you define a pump?
A very simple question you might think especially for a hydraulic professional.
This was the simple question posed by a young graduate on Linkedin. Over 3 months later with over 100 comments it is still being debated with some veracity.
I was always taught that pumps produce flow and pressure is a restriction of flow. A challenge to that view is that there must be a force or pressure to move any mass, therefore fluid being a mass means that if there is flow from a pump there must be pressure. I have to agree to this observation and is irrefutable.
So why don’t hydraulic people acknowledge this pressure when we know that there must be some pressure , no matter how small?.
Some things we are taught are often accepted without challenge because with a subject like hydraulics at the beginning one is trying to cope with the overall understanding without thinking of challenging the basics. These concepts eventually become accepted and fixed in one’s own mind until one day someone challenges that view.
I will give you my opinion why we ignore/deny that pumps produce pressure. I believe that by acknowledge this inbuilt load/pressure in our” definition of a pump” will it make the concept clearer or more confusing especially to those with little knowledge of the subject? I believe the latter but more than that what extra benefit does it provide to the hydraulic engineer? I suspect very little.
We know that pumps have different efficiency values. We acknowledge this in our pump power calculations. No pump is 100% and it will be reduced by friction in bearings, running gear and of course the energy needed to push the oil through the pump (also leakage). In hydraulics we factor this into our calculations and because it is a relatively small force value we often tend to ignore it. Remember that hydraulics is used to move or rotate very large loads and as a percentage of these loads the force to push the oil through the pump is very small indeed.
In future then, now that I have given more thought to this issue, how will I describe a pump? I will likely make some adjustments based on whom I am speaking to. Those starting out in hydraulic or struggling with very basic knowledge I will stick to “pumps provide flow” and for those that require a more precise definition: “A pump is a device that forces fluid through its mechanical parts to produce flow” This is wordier but I think a more accurate definition but not sure that it changes much!
Very best
Bob
A very simple question you might think especially for a hydraulic professional.
This was the simple question posed by a young graduate on Linkedin. Over 3 months later with over 100 comments it is still being debated with some veracity.
I was always taught that pumps produce flow and pressure is a restriction of flow. A challenge to that view is that there must be a force or pressure to move any mass, therefore fluid being a mass means that if there is flow from a pump there must be pressure. I have to agree to this observation and is irrefutable.
So why don’t hydraulic people acknowledge this pressure when we know that there must be some pressure , no matter how small?.
Some things we are taught are often accepted without challenge because with a subject like hydraulics at the beginning one is trying to cope with the overall understanding without thinking of challenging the basics. These concepts eventually become accepted and fixed in one’s own mind until one day someone challenges that view.
I will give you my opinion why we ignore/deny that pumps produce pressure. I believe that by acknowledge this inbuilt load/pressure in our” definition of a pump” will it make the concept clearer or more confusing especially to those with little knowledge of the subject? I believe the latter but more than that what extra benefit does it provide to the hydraulic engineer? I suspect very little.
We know that pumps have different efficiency values. We acknowledge this in our pump power calculations. No pump is 100% and it will be reduced by friction in bearings, running gear and of course the energy needed to push the oil through the pump (also leakage). In hydraulics we factor this into our calculations and because it is a relatively small force value we often tend to ignore it. Remember that hydraulics is used to move or rotate very large loads and as a percentage of these loads the force to push the oil through the pump is very small indeed.
In future then, now that I have given more thought to this issue, how will I describe a pump? I will likely make some adjustments based on whom I am speaking to. Those starting out in hydraulic or struggling with very basic knowledge I will stick to “pumps provide flow” and for those that require a more precise definition: “A pump is a device that forces fluid through its mechanical parts to produce flow” This is wordier but I think a more accurate definition but not sure that it changes much!
Very best
Bob
Wednesday, 18 May 2011
Which pump?
When I first started out designing systems we had the choice between vane or piston pumps.
Gear pumps at that time were rather frowned upon and not readily available.
I haven’t used a fixed vane pump since I can remember and piston pumps only for occasions when I need high pressure or my customer is keen on quality/reliability rather than cost.
There is no doubt that the gear pump has become more popular but maybe our hydraulic group may not all agree with this trend.
Personally I will use a variable vane for those systems where I need to hold pressure for long periods with low pressure. The piston pump for higher pressure, long periods on pressure with big flows. The gear pumps for everyday low duty and competitively priced.
You will all have your own ideas for selecting the ideal pump and I would love to hear your views.
Very best
Bob
Gear pumps at that time were rather frowned upon and not readily available.
I haven’t used a fixed vane pump since I can remember and piston pumps only for occasions when I need high pressure or my customer is keen on quality/reliability rather than cost.
There is no doubt that the gear pump has become more popular but maybe our hydraulic group may not all agree with this trend.
Personally I will use a variable vane for those systems where I need to hold pressure for long periods with low pressure. The piston pump for higher pressure, long periods on pressure with big flows. The gear pumps for everyday low duty and competitively priced.
You will all have your own ideas for selecting the ideal pump and I would love to hear your views.
Very best
Bob
Saturday, 7 May 2011
Tuesday, 3 May 2011
Hydraulic Oil
We talk a great deal about pumps, valves, pipe-work and other hardware but give little mention about the lifeblood of the system, that of course is hydraulic oil.
This is vitally important to the life and performance of the system and come in many varieties.
When I started my career we were told to say that any oil is ok as long as it is mineral ISO 32 hydraulic oil and I suppose it is still the most common choice today.
But we also have applications where this is not suitable and today we have more choices and better oils to better suit demanding conditions.
The environment has become a greater issue and it is very important we do not contaminate the soil or waterways. Rape seed oils were common but now there are some very good synthetic oils that not only protect the environment but are better suited to the components.
Fire has long been an issue with fire resistant fluid. Water glycols must be used with overhead tanks due to specific gravity and to avoid cavitation. However there are fluids that can be used in the same way as normal oil that give similar protection.
Skydrol is used in aerospace systems and will destroy most types of rubber and plastic and need careful selection of pumps and valves.
It might be that the system is working in very hot or very cold environments and will need a suitable grade of oil that will end up with the ideal viscosity at the working temperature.
There are also oils that will not degrade when run at higher temperatures.
With mineral oils, I do not like temperatures above 50centigrade (comfortable to the touch).
Higher temperatures can cause many problems with seals, varnishing and eventually breakdown of the additives that protect the systems.
I would like to hear about oils that our group has used and maybe stories of problems when unsuitable oil is used.
Cheers
Bob
This is vitally important to the life and performance of the system and come in many varieties.
When I started my career we were told to say that any oil is ok as long as it is mineral ISO 32 hydraulic oil and I suppose it is still the most common choice today.
But we also have applications where this is not suitable and today we have more choices and better oils to better suit demanding conditions.
The environment has become a greater issue and it is very important we do not contaminate the soil or waterways. Rape seed oils were common but now there are some very good synthetic oils that not only protect the environment but are better suited to the components.
Fire has long been an issue with fire resistant fluid. Water glycols must be used with overhead tanks due to specific gravity and to avoid cavitation. However there are fluids that can be used in the same way as normal oil that give similar protection.
Skydrol is used in aerospace systems and will destroy most types of rubber and plastic and need careful selection of pumps and valves.
It might be that the system is working in very hot or very cold environments and will need a suitable grade of oil that will end up with the ideal viscosity at the working temperature.
There are also oils that will not degrade when run at higher temperatures.
With mineral oils, I do not like temperatures above 50centigrade (comfortable to the touch).
Higher temperatures can cause many problems with seals, varnishing and eventually breakdown of the additives that protect the systems.
I would like to hear about oils that our group has used and maybe stories of problems when unsuitable oil is used.
Cheers
Bob
Sunday, 1 May 2011
Valve leakage
I hear a lot of myths surrounding leakage.
There have been many times when I have been asked to design a system where the client wants to hold a load almost indefinitely (leak free).
Most sliding valves need some leakage for lubrication due to metal to metal contact. Poppet valves or seated valves are as good as the next particle of contamination that lodges under the seat. Soft seated valves can give better chance of success but are less available.
The all ports blocked CETOP valve can cause problems with creeping rams. P port being close to A&B with T ports on the far side will allow pressure to A&B and send the ram out. The only remedy is either install a new valve or change to a P to T and a PO check.
Also in the attempt to lock off a load (hydraulicaly) can be dangerous. Any oil passing over the piston and with no route to tank will cause pressure intensification that might be well above the maximum allowed.
I have never relied on valves being leak free and always play safe.
Love to hear your views
Bob
There have been many times when I have been asked to design a system where the client wants to hold a load almost indefinitely (leak free).
Most sliding valves need some leakage for lubrication due to metal to metal contact. Poppet valves or seated valves are as good as the next particle of contamination that lodges under the seat. Soft seated valves can give better chance of success but are less available.
The all ports blocked CETOP valve can cause problems with creeping rams. P port being close to A&B with T ports on the far side will allow pressure to A&B and send the ram out. The only remedy is either install a new valve or change to a P to T and a PO check.
Also in the attempt to lock off a load (hydraulicaly) can be dangerous. Any oil passing over the piston and with no route to tank will cause pressure intensification that might be well above the maximum allowed.
I have never relied on valves being leak free and always play safe.
Love to hear your views
Bob
Subscribe to:
Posts (Atom)