replacing a nitra a24050dp-m air cylinder with hydraulic 5

[booshambo]

Hello,
I don't know where to look for low duty cycle low pressure hydraulic components.

I have an application that uses Nitra A24050DP-M and Nitra A24030DP-M air cylinders at about 100 or 110 Psi.

I'd like to move from air to hydraulic in hopes that
1. I can make the retracted system stiffer to better locate the driven stuff.
2. I can save weight.

I (in my hydraulic ignorance) am unable to find such smallish low pressure cylinders off the shelf. The parts above are about 11.5 inches at rest with a 5 inch stroke
and 9.5 inches at rest with a 3 inch stroke. They are rated to 250 PSI.

At present I believe the extension force required is less than 25 or 30 pounds.
Duty cycle is roughly once per hour while the machine is in operation.

Any suggestions as to where to look for such small gear would be welcome, as well as any "It'll never work" stories.

Thanks very much,

b

 

[PNachtwey]

The force you need is so small you can't justify a hydraulic system. Look at using an electro-mechanical cylinder like an Exlar or similar.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

IFPE Hall of Fame Member

 

[BrianPetersen]

Hydraulics for such low pressures aren't used. Not saying you can't ... it just isn't done. You'll end up using components designed for much higher pressures, and just running them at a low pressure. Not worthwhile.

You won't save weight by the time you include the requisite hydraulic power pack with pump, motor, oil tank, etc., and the hydraulic hoses, pipes, and fittings will weigh far more (and be much less flexible and harder to work with) than the dinky little plastic hoses that are all you need for pneumatics. And the inevitable oil leaks will annoy you to no end.

I totally agree on the electric-actuator suggestion.

Parker has many different styles:

https://ph.parker.com/ca/en/electric-cylinders

Tolomatic is another name that I've seen:

https://www.tolomatic.com/products/...-cylinders-for-pneumatic-cylinder-replacement

 

[booshambo]

Thanks much for the replies. I am perhaps remiss in not having fully explained the surrounding constraints, which are what I feel are backing me into the hydraulic corner.

The basics of the application are three cylinders each actuating one of three landing gear on a very small aircraft. The current system works but has these uncomfortable (imo) characteristics.
1. It requires a pair of air pumps (for redundancy, motor/pump being considered an unacceptable single failure point) and a tank which must admit to frequent draining of water.
2. Air being compressible the stiffness of the system in resisting gear extension during maneuvering flight requires some kind of alternative uplock which becomes another point
of critical reliability.
A huge advantage to this system is that it is operational and seems to work, i.e. the only leak based failure I'm aware of was mitigated by being noticed in time and a precautionary landing
saved the day. This leak was too large to be overcome by the onboard compressors. I'm unaware of the system having any leak problems subsequent to this initial flight test event.

Reliability in prior art
-------------------------

Industry practice seems to consider gearboxes (for speed reducing or routing motor drives) and linkages to be reliable (i.e. they needn't enjoy/suffer redundancy. Motor failure and the
absence of electricity however are not acceptable reasons to have to land gear up. It is fairy common to have some mechanism to manually rotate a motor or the motor speed reduction output
to effect emergency extension. Some hydraulic systems (either electric/hydraulic or engine driven pump hydraulic) rely on manual pumps to effect an alternate extension means. Systems which
rely on the pilot moving a lever to mechanically extend and retract the gear have been deployed with no backup.

Back to my problem
------------------

Given the above context my tastes run to simplicity and low weight. Electric actuators put three motors beyond human reach with a material increase in the probability of needing to land with
one wheel partially extended which is worse by far (in this airplane) than landing with all three up.

It has occurred to me that automotive hydraulic clutches (master/slave actuators) are an example of fluid power to use human prime mover over a distance without the routing complication of
mechanical linkages. Current clutch systems have a very fancy throwout bearing that seems to combine the slave cylinder with the bearing, but in the old days there was a
cylinder at the clutch that just operated a pivot bar through the bell housing.

Some boats use a similar approach to steering, with what looks like a hydraulic swash plate piston pump controlling/driving a double acting cylinder at the stern. Some models are powered only by
physical rotation of the steering wheel, others have an additional pressure pump.

My hope is to have a human powered reversible pump that extends or raises the gear. This lets me get rid of the onboard compressors and tank, the fluid (vs air) may obviate the need for up locks,
and since the extend or retract sides of the three pistons are all joined the fact that one cylinder is 3 inch stroke and two of them are 5 inch doesn't even have to have a limit switch
issue as would be present in the electric actuator case. ( Not a big problem that, but has to be part of the failure analysis).

Given all this maybe I should be asking for examples of hand powered cylinders of the 3 or 5 inch variety.

The elegance of using a gear pump that is cranked one way for up and the other for down seems very attractive to me for completely removing any need for valving (no spool valve or similar needed) but
from what I can glean there's always a path internally to vent bearing "blow by" to tank which makes me wonder if I can just crank it backwards to reverse the flow. All the "how to
reverse" info I can find require wrenches...

Thanks for the prior remarks with which I fully agree.

Any further thoughts insights or warnings quite welcome.

Regards,
b

 

[LittleInch]

That's a bit different.

My limited understanding of aircraft landing systems is that they lock into position to avoid having a constant pressure and yes, you need very reliable systems with redundancy.

I thought lowering of the gear had to be possible by weight alone? Or is that just passenger planes?

There is an aircraft forum which may actually be better to advise.

Are you using extension of the piston to lower the gear and retraction brings it back up?

A hand pump arrangement is easier for hydraulics for sure, but could you not use some sort of manual retraction?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SwinnyGG]

How heavy is this gear that you need hydraulics at all??

It seems to me that if this plane really is that small, there's probably a pure mechanical solution (ie levers/bowden cables) which would get you home.

 

[booshambo]

Some more clarifications - thanks for the interest.


> My limited understanding of aircraft landing systems is that they lock into position to avoid having a constant pressure and yes, you need very reliable systems with redundancy.
They typically (like almost all of them) lock down with an overcenter linkage which ensures that side and drag loads can't make the gear fold up. Reteaction must first "un overcenter" this
link. Up locks are often of a latch variety and in this case that or originally (earlier model) uplock is effected solely by air pressure in the retraction cylinders.


> I thought lowering of the gear had to be possible by weight alone? Or is that just passenger planes?>
I'm unaware of regulations mandating gravity fall for any airplanes. I'm certain it is not mandated in this case. There are typically emergency procedures for gear extension failure due to
various events within the published set of failure modes. In the case of the air cylinder system mentioned above the emergency procedure is to go immediately to "land gear up". This is
typically the last solution for all aircraft with finite fuel capacity. That solution is expensive (if it occurs) but not very dangerous.

> Are you using extension of the piston to lower the gear and retraction brings it back up?
Yes. A cylinder on each of three landing gear legs.

> A hand pump arrangement is easier for hydraulics for sure, but could you not use some sort of manual retraction?
The massive advantage of fluid drive here is the routing of the load path via hydraulic lines versus linkages be they Bowden cables, closed loop cables (pull pull) or push pull and torque rods.
In addition a 1:1 linkage suffers from presenting the user with a 60 lbs force knob in the case of a manual Bowden cable system, hence some kind of leveraging device is required within a
24 in wide cockpit. Hydraulics offer the possibility of getting suitable ratios from just changing the relative cylinder bores.


> It seems to me that if this plane really is that small, there's probably a pure mechanical solution (ie levers/bowden cables) which would get you home.
It is certainly that small (16.5 feet long) but I hear you on the mechanical solution. I though of bowden cables first but without redesign of the gear itself the 60 or so lbs force on the
user handle means some kind of ratchet mechanism and there's preccious little room to make the min radius constraints on those that can do that. The weight of the gear is not the entire
issue of course as it has to operate in a wind environment of 125 mph or so which does obvious and not so obvious things to the forces required. Nose gear for example has to extend against the wind.


So bottom line is I still would dearly love to have a gear pump (or trochoid?) I can operate by hand that implements clockwise down and anticlockwise up and a few tiny cylinders that fit in
the current form factor thus allowing me to ditch the entire pair of compressors and tank and uplocks so I can save some weight and increase reliability.
I sense the answer is going to be something like "you have a lathe right?"




Regards,
b

 

[btrueblood]

The first problem with the bi-directional gear pump you describe is - how do you fill the system with fluid, and what happens if you have a leak? Get a bubble in a line, and the pump will aspirate air and stop pumping. Might work if a pressurized accumulator was in the line to provide make-up fluid, but again filling is tricky. 2nd problem is that most pumps are built with inline check valves, i.e. don't operate bidirectionally, sounds like you'd have to build that as a custom job.

Simpler might be to have a uni-direction pump with a directional spool valve on it (manually actuated it sounds like you want). Advantage is that the pump intake is an atmospheric pressure tank. Hmm...that might not be what you want anyway, if you plan on any aerobatics with negative G maneuvers (oil spill through the tank vent), and maybe a low pressure piston accumulator will be needed.

Bimba makes a line of lightweight aluminum cylinders for low pressure (250-500 psi) hydraulics. I'm sure there are others, maybe check the Parker and Clippard websites.

 

[booshambo]

Hello - Thanks for the Bimba and etc suggestions! They look to be the proper next step.

My naive hope was that a bi-rotational hand operated rotary would see the extend side of the three pistons as the load and the retract sides as tank so I could get by with
an expansion tank/accumulator of some description dictated by empirical results. Very hand wavy on my part re cavitation on the suction side and all of that.
Still hopeful for the hydraulic equivalent of a cable wound around a drum with pullright / pullleft on pulleys to the load... Maybe an accumulator as a tension spring.

Inverted is certainly relevant but conventional hydraulic manual (pedal) brakes operate routinely in (or rather subsequent to) +/- 9 g settings. Anyway I think having a tank at 1 atmosphere is not a real issue if it is made to not leak through the vent system.
During use (landing) the system is experiencing no negative G at all, one hopes.

Anyway thanks again for the pointers. I'm also off looking at trochoid pumps on motorcycles. Might be the proper size and features.

Regards,
b

 

[hydtools]

Example of power units including reversible pumps.

Ted

 

[booshambo]

--- 0 ---
<From hydtools in part...>
Example of power units including reversible pumps.
--- 0 ---

Thanks ever so much for that - seriously useful. First appnote of Christmas!

b

 

[Jacc]

There are a lot of producers of these small HPU's like the one hydtools posted and most of them have a bidirectional version in their range. Google responds well to Mini Powerpack.

Here are a few:
Oilsystem (Rexroth)
Hydronit
Brevini (Hydra-App)
Bucher (Monarch)

 

[booshambo]

There are a lot of producers of these small HPU's like the one hydtools posted and most of them have a bidirectional version in their range. Google responds well to Mini Powerpack.

Here are a few:
Oilsystem (Rexroth)
Hydronit
Brevini (Hydra-App)
Bucher (Monarch)

Thanks for these - Several days of searching and I had seen only Rexroth in passing. I never would have thought to try Mini Powerpack.

Regards,
b

 

[Jacc]

You're welcome. Here are some more

HPI
Gazzera
Hydac

 

[srini1785]

I am not an aeronautic engineer and my understanding on the subject is very limited. In case you don't want a messy (Sorry for the word) hydraulic system like a pump, tank etc but would still like to generate the type of forces a hydraulic system produces then you can look at an Air oil booster.

http://www.fluidforce.it/pdf/18 Moltiplicatori_en 03_2013.pdf

Few other companies also make it. Operation is simple : Control the air pressure to control the Hydraulic pressure output. The advantage is that the Hydraulic cylinder is pretty compact and easy to control.