I am do the sums to calculate the required size of hydraulic ram need to
drive a transom hung rudder.

I read somewhere that it is preferable to use two rams (one on each side )
to minimise the amount of twisting forces on a rudder.

If I decide to go for this method, is it acceptable to divide the torque
requirement by two.

I.E
If I need a 90KgM torque to drive the rudder, can I size the rams for 2 x
45, or should I use 2 x 90 ?

garry

If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod. For
example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.

garry crothers wrote:

I am do the sums to calculate the required size of hydraulic ram need to
drive a transom hung rudder.

I read somewhere that it is preferable to use two rams (one on each side )
to minimise the amount of twisting forces on a rudder.

If I decide to go for this method, is it acceptable to divide the torque
requirement by two.

I.E
If I need a 90KgM torque to drive the rudder, can I size the rams for 2 x
45, or should I use 2 x 90 ?

garry



--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Glenn Ashmore" wrote in message
news:rJiDb.4799$JD6.1854@lakeread04...
If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod.


Excuse my ignorance, but how can I tell if the cylinder is double action or
single. I've just looked at the Vetus catalogue and cant see any reference
to single or double?


For
example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.

I kinda thought that it would just be as simple as divide by 2...



thanks again
garry

The Vettus system is a double acting "balanced" cylinder. Balanced
cylinders have the rod going all the way through so the pressure is the
same in both directions. In that case you can divide by 2.

Keep in mind though that if you use two cylinders you need to double the
capacity of the pump to get the same number of hard over turns.

garry crothers wrote:

"Glenn Ashmore" wrote in message
news:rJiDb.4799$JD6.1854@lakeread04...

If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod.



Excuse my ignorance, but how can I tell if the cylinder is double action or
single. I've just looked at the Vetus catalogue and cant see any reference
to single or double?


For

example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.


I kinda thought that it would just be as simple as divide by 2...



thanks again
garry



--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

On Mon, 15 Dec 2003 08:57:02 -0500, Glenn Ashmore
wrote:

The Vettus system is a double acting "balanced" cylinder. Balanced
cylinders have the rod going all the way through so the pressure is the
same in both directions. In that case you can divide by 2.

No. But your previous answer was good, so I am puzzled!

Brian W

Keep in mind though that if you use two cylinders you need to double the
capacity of the pump to get the same number of hard over turns.

garry crothers wrote:

"Glenn Ashmore" wrote in message
news:rJiDb.4799$JD6.1854@lakeread04...

If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod.



Excuse my ignorance, but how can I tell if the cylinder is double action or
single. I've just looked at the Vetus catalogue and cant see any reference
to single or double?


For

example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.


I kinda thought that it would just be as simple as divide by 2...



thanks again
garry

Brian Whatcott wrote:

On Mon, 15 Dec 2003 08:57:02 -0500, Glenn Ashmore
wrote:


The Vettus system is a double acting "balanced" cylinder. Balanced
cylinders have the rod going all the way through so the pressure is the
same in both directions. In that case you can divide by 2.


No. But your previous answer was good, so I am puzzled!

My original post assumed a standard double acting cylinder. These have
the rod attached to one side of the piston. In these, the displacement
on the rod side is reduced by the volume of the rod and the pressure
face of the piston is reduced by the cross sectional area of the piston.
That means you get less force on the pull stroke than on the push for
the same fluid pressure. Also you get more movement for the same volume
of fluid. That makes them hard to use for steering unless they are used
in opposing pairs like on heavy earth moving equipment.

A balanced or "steering" cylinder has the rod continue through the
piston and out the other end of the cylinder. This end of the rod
usually carries no force but makes the displacement and pressure face
the same on the two sides so the force is the same in both directions.

The down side is that you have two sets of seals that will leak someday
rather than just one.


--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"garry crothers" wrote in message
...
Excuse my ignorance, but how can I tell if the cylinder is double action
or
single. I've just looked at the Vetus catalogue and cant see any
reference
to single or double?

I think all Vetus cylinders are double action. If you see one hose nipple on
each end of the cylinder, they definately are.

Besides, I fail to see how two rams can prevent any twist anywhere in the
rudder system.

Meindert

"Meindert Sprang" wrote in message
...
"garry crothers" wrote in message
...
Excuse my ignorance, but how can I tell if the cylinder is double action
or
single. I've just looked at the Vetus catalogue and cant see any
reference
to single or double?

I think all Vetus cylinders are double action. If you see one hose nipple
on
each end of the cylinder, they definately are.

Besides, I fail to see how two rams can prevent any twist anywhere in the
rudder system.

Meindert

I believe that the advantage with two rams is that, because one cylinder
pushes aft, while the other pulls forward with the same force, the only
force on the rudder stock is the desired twisting, and no side loading is
imposed, as the case would be with the use of a single cylinder.

But then again I am no expert,

garry

Doing a really rough vector analysis it appears that with the rudder
within 15º of midships there is very little difference in the turning
force between a single and a double cylinder setup but because one
cylinder is pushing out while the other is pulling in, the pressure on
the pintle is somewhat less. Not sure it is enough to justify a second
cylinder with the extra cost, hose and fittings though.

Not much to be gained in redundancy either. The most likely point of
failure is not the cylinders but the pump, hose and fittings.

garry crothers wrote:

"Meindert Sprang" wrote in message
...

"garry crothers" wrote in message
...

Excuse my ignorance, but how can I tell if the cylinder is double action

or

single. I've just looked at the Vetus catalogue and cant see any

reference

to single or double?

I think all Vetus cylinders are double action. If you see one hose nipple

on

each end of the cylinder, they definately are.

Besides, I fail to see how two rams can prevent any twist anywhere in the
rudder system.

Meindert


I believe that the advantage with two rams is that, because one cylinder
pushes aft, while the other pulls forward with the same force, the only
force on the rudder stock is the desired twisting, and no side loading is
imposed, as the case would be with the use of a single cylinder.

But then again I am no expert,

garry



--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

On Mon, 15 Dec 2003 14:24:24 -0000, "garry crothers"
wrote:


"Meindert Sprang" wrote in message
...
"garry crothers" wrote in message
...
Excuse my ignorance, but how can I tell if the cylinder is double action
or
single. I've just looked at the Vetus catalogue and cant see any
reference
to single or double?

I think all Vetus cylinders are double action. If you see one hose nipple
on
each end of the cylinder, they definately are.

Besides, I fail to see how two rams can prevent any twist anywhere in the
rudder system.

Meindert

I believe that the advantage with two rams is that, because one cylinder
pushes aft, while the other pulls forward with the same force, the only
force on the rudder stock is the desired twisting, and no side loading is
imposed, as the case would be with the use of a single cylinder.

But then again I am no expert,

garry

Quite! :-) (no offence)

Brian W