Hi all,

This is driving me nuts. I have a vertical door that opens out and
down to horizontal like a dishwasher door. I have been trying to draw
this in CAD and getting nowhere fast.

I have limited room to mount a gas spring and I also have a gas spring
that will fit the available space. I have searched and drawn a blank
on this calculation. I have found plenty to calculate the force.

Can someone please point me at a website or spreadsheet where I can
enter in the open length (11-3/4" centers) the stroke (4") and
opening (90degrees) and have it tell me where to mount the brackets.
The opening angle is not critical so it could be between 80 and 90.
The hinge is on the outside of the 1/2" thick door. Not conventional
but I am working with what I've got. grin

ASCII art of the set up...
+----++-+
| 1" || |
| Sq.|| |
+----+| |
Max | | -- Door opens this way --
vertical | | to 90 degrees flat
space in | |
here | |
between | |
top and | |
bottom | |
frame | |
is 12.5" | |
| |
+----+| |
| 1" || |
| Sq.|| |O -- Hinge
+----++-+

Thanks for any help.

Dave

The Davester wrote:
Hi all,

This is driving me nuts. I have a vertical door that opens out and
down to horizontal like a dishwasher door.

(...)


Do you want the gas spring to exert a
force to close the door or to open it?

I assume you want the door to be self - closing.

Were I you, I would cheat.
create a layer that shows the door fully open,
another layer showing the door fully closed.

Show the body of the gas spring suspended upside
down via a shoulder bolt to the top of the inside of
the opening, nearly fully compressed in
the "fully open" layer. Mounting height does not
matter yet so group the gas spring with the shoulder
bolt so you can move them both in a little while.

Rotate the gas spring so that it's bottom end protrudes
a few degrees into the opening.

Sketch a 4.5" high door bracket near the door hinges
oriented 90 degrees to the door and move the gas
spring / shoulder bolt combination -Z.

Show the piston of the gas spring attached near
the top of the door bracket via another shoulder bolt.

In the "fully closed" layer, rotate your gas
spring so that it remains constrained to the
shoulder bolt that is attached to the door bracket.

Tweak the placement of the shoulder bolts so
that you show the gas spring with proper extension.

Let the computer do the math.


--Winston

On Nov 11, 3:47 pm, Winston wrote:

Hi Winston, thanks for the reply but it is not quite that simple. The
gas spring has to compress almost fully during about 80% of the door
movement when closing, then it passes a toggle point and the gas
spring begins to extend again to provide the holding pressure.

I finally decided to give up on the CAD and mounted the top as high as
possible. I then clamped the door bracket to the door and tested. I
kept moving the bracket closer to the hinge edge of the door until I
got to that compression point where the end almost hits the tube.
That's where I drilled the holes. I then went back and filled in the
other 6 holes I had started with when I had thought it was simple
installation. LOL

This is the second gas spring I have had to mount in about 3 weeks and
I sure would like to find a method that works better than the sneaking
up on it one. I found a website with some lengthy and complex
Trigonometry and thought it was over complicated. I will go back and
look at that again.

Dave

On Nov 12, 10:20*am, "Dave, I can't do that"
wrote:
On Nov 11, 3:47 pm, Winston wrote:

Hi Winston, thanks for the reply but it is not quite that simple. The
gas spring has to compress almost fully during about 80% of the door
movement when closing, then it passes a toggle point and the gas
spring begins to extend again to provide the holding pressure....

Dave

I solved a similar problem with the hydraulic cylinder geometry on my
front end loader by drawing the parts to scale and using a ruler
marked with the extended and retracted lengths for the hypotenuse.

I'm also messing about with a gas spring. Can you post the web site
you speak of?

I found a website with some lengthy and complex
Trigonometry and thought it was over complicated. I will go back and
look at that again.

Dave

Dave, I can't do that wrote:
On Nov 11, 3:47 pm, Winston wrote:

Hi Winston, thanks for the reply but it is not quite that simple. The
gas spring has to compress almost fully during about 80% of the door
movement when closing, then it passes a toggle point and the gas
spring begins to extend again to provide the holding pressure.

Ah! I didn't know that.

(...)

I then went back and filled in the
other 6 holes I had started with when I had thought it was simple
installation. LOL

I call that 'successful approximation', when I do it.

This is the second gas spring I have had to mount in about 3 weeks and
I sure would like to find a method that works better than the sneaking
up on it one. I found a website with some lengthy and complex
Trigonometry and thought it was over complicated. I will go back and
look at that again.

If you can capture the equations on a spreadsheet, life could get
very much easier, especially if solved examples are available.

--Winston

On Wed, 12 Nov 2008 07:20:25 -0800, Dave, I can't do that wrote:
On Nov 11, 3:47 pm, Winston wrote:
Hi Winston, thanks for the reply but it is not quite that simple. The
gas spring has to compress almost fully during about 80% of the door
movement when closing, then it passes a toggle point and the gas spring
begins to extend again to provide the holding pressure.

I finally decided to give up on the CAD and mounted the top as high as
possible. I then clamped the door bracket to the door and tested. I kept
moving the bracket closer to the hinge edge of the door until I got to
that compression point where the end almost hits the tube. That's where
I drilled the holes. I then went back and filled in the other 6 holes I
had started with when I had thought it was simple installation. LOL

This is the second gas spring I have had to mount in about 3 weeks and I
sure would like to find a method that works better than the sneaking up
on it one. I found a website with some lengthy and complex Trigonometry
and thought it was over complicated. I will go back and look at that
again.

You might see if the following works out. This procedure would put
the lower pivot point about 4.2" up and 1.8" in from the hinge vertex.

The ascii diagram outlined below is like the diagram at
http://cabinet-gas-spring.spring-man...er.com/f1.html (end of
http://www.physicsforums.com/showthread.php?t=116365 links to it)
except reversed top to bottom, to match your diagram. In
following, door hinge is at O, and gas-spring pivot points are at
T and B or B'. Three door positions are shown. At OD, the
spring is at minimum length and the door is open alpha
radians (say 18 degrees, for compression over 80% of 90 degrees).
At OD', the spring is at maximum length and the door is open 90
degrees. At OD" the door is closed.


D"
| D
|
T |
|
|
|
|
|
|
B |
| K
|
| B'
|
|
O-------K'-------------D'

Draw the following lines on the diagram: TBO, OD", OKD, OB', and
OK'D', and label the angles and distances as described below.

Let distance d = BT = minimum length of gas spring; e = B'T =
maximum length of gas spring; v = OK = OK' = y-axis distance of
lower pivot's bracket from door's hinge; w = KB = K'B' = x-axis
distance of lower pivot from hinge side of door; u = x-axis
distance from top pivot T to y axis. I'm supposing that d, e, u,
and alpha are given and we want to solve for v and w.

From before, alpha = D"OD = 18 degrees.
Let beta = BOB' = TOB' = pi/2 - alpha = 72 degrees.
Let phi = OTB', eta = OBB' = OB'B = (pi-beta)/2 = 54 deg.
Let gamma = TBB' = pi-eta = 126 deg., and nu = TB'B.

By law of sines, sin gamma / e = sin nu / d, so nu = arcsin ((d
sin gamma)/e). phi = pi-gamma-nu. Let eps = BOK in right
triangle with sides r, v and w. sin beta / e = sin phi / r,
r = e sin phi / sin beta. u = (d+r) sin (eps-alpha) so
eps-alpha = arcsin(u/(d+r)) and eps = alpha + arcsin(u/(d+r)).
Finally, w = r sin eps and v = r cos eps.

For example, if d = 7.75", e = 11.75", u = door thickness +
cyl. diam/2 = 1", and alpha = 18 degrees = .314... radians, we
get: beta = pi/2 - alpha ~ 1.26, eta = (pi-beta)/2 = .942,
gamma = pi-eta = 2.2, nu ~ .562, phi ~ .38, r = 4.58,
eps = .3956 ~ 23 degrees, w = 1.76", v = 4.22".


--
jiw

On Nov 12, 12:00 pm, Dan@ (Dan ) wrote:

Hi Dan,

The link is the second one James shows in his reply.

I'm also messing about with a gas spring. Can you post the web site
you speak of?

On Nov 13, 1:15 am, James Waldby wrote:

You might see if the following works out. This procedure would put
the lower pivot point about 4.2" up and 1.8" in from the hinge vertex.

Hi James, you obviously think I am waaay smarter than I am.

Thanks for doing that but I really have no idea what all that means.
Just a good-boy old machinist here.

I had already found both the links though but thanks for searching and
posting them.

Dave

On Thu, 13 Nov 2008 08:13:42 -0800, Dave, I can't do that wrote:

On Nov 13, 1:15 am, James Waldby wrote:

You might see if the following works out. This procedure would put the
lower pivot point about 4.2" up and 1.8" in from the hinge vertex.

Hi James, you obviously think I am waaay smarter than I am.

Thanks for doing that but I really have no idea what all that means.
Just a good-boy old machinist here.

I had already found both the links though but thanks for searching and
posting them.

Do you have a C compiler? If not, maybe someone can compile
this program for you -- http://pat7.com/js/c/gas-spring-calc.c
or perhaps someone will translate it to a spreadsheet.

The program can prompt you to enter the necessary numbers, or
can accept parameters on the command line and use some default
values for others. For example, ./gas-spring-calc 13 21 2.2
produces:

With inputs 13.00 21.00 2.20 18.0 90.0,
the lower pivot is up 8.20 and in 3.61, and
the upper pivot is up 21.85 and in 2.20,
relative to the door hinge.

--
jiw

On Nov 13, 1:54 pm, James Waldby wrote:

Hi James,

Do you have a C compiler? If not, maybe someone can compile

Yes I do have a C-compiler, well C++ and will give it a shot. Thanks.
Actually I may re-write it in Delphi as I am more comfortable with
that and will post the code back here.

Thanks

Dave