I have just launched on new forum on Linkedin.
The purpose of the group is to help and support each other on issues relating to Fluid Power.
Due to the platform used, where the only method of communication is print extra effort must be made to ensure that the message sent to the recipient and the group should appear friendly, helpful and positive to the discussion.
We hope to have fun, make good friends and learn about other members Fluid power experiences.
Please come and join us.
The name of our forum is "Hydraulic help"
Cheers
Bob
Friday, 17 June 2011
Wednesday, 15 June 2011
Controlling Speed
There are many ways to control the speed of a hydraulic ram or motor.
We could use a pump that delivers a variable flow (variable piston or vane pump)
The easiest way is to fit a flow control valve but the question comes where do you fit it?
Meter out: This is where a valve is fitted to each leg of the mover. The arrow for free flow should point towards the mover port. The flow is restricted on the return leg back to the control valve. This will prevent the load running ahead of the oil supply and keep the system stiff. However if screwed down too far can cause some intensification on the annular side of the ram.
Meter in: Good when the mover is pushing against a load but not if the load is trying to pull the ram. Imagine pushing a car up a hill. The best place to push is behind the car. As the car reaches the summit you need to move to the front to stop the car running away. The valve should be fitted with the free flow arrow pointing away from the mover towards the control valve.
Bleed off: This can be fitted on any leg of the mover or the main pressure line.
The valve is piped to tank and bleed oil off the main flow. This can be problematic and will not give a wide range of speed control. One main benefit is that it is less prone to generate heat.
Flow control on Pressure inlet: The valve would be a simple needle valve type.
Will cause heat generation. Excess oil will pass across the relief valve.
If you are unsure what is the best way for your system please send me an email.
I will write more on this subject including, pressure and temperature compensation and Load Sensing.
See more on www.hydraulicbrain.com
Cheers
Bob
We could use a pump that delivers a variable flow (variable piston or vane pump)
The easiest way is to fit a flow control valve but the question comes where do you fit it?
Meter out: This is where a valve is fitted to each leg of the mover. The arrow for free flow should point towards the mover port. The flow is restricted on the return leg back to the control valve. This will prevent the load running ahead of the oil supply and keep the system stiff. However if screwed down too far can cause some intensification on the annular side of the ram.
Meter in: Good when the mover is pushing against a load but not if the load is trying to pull the ram. Imagine pushing a car up a hill. The best place to push is behind the car. As the car reaches the summit you need to move to the front to stop the car running away. The valve should be fitted with the free flow arrow pointing away from the mover towards the control valve.
Bleed off: This can be fitted on any leg of the mover or the main pressure line.
The valve is piped to tank and bleed oil off the main flow. This can be problematic and will not give a wide range of speed control. One main benefit is that it is less prone to generate heat.
Flow control on Pressure inlet: The valve would be a simple needle valve type.
Will cause heat generation. Excess oil will pass across the relief valve.
If you are unsure what is the best way for your system please send me an email.
I will write more on this subject including, pressure and temperature compensation and Load Sensing.
See more on www.hydraulicbrain.com
Cheers
Bob
Sunday, 12 June 2011
One pump or two?
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
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
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
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