FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups 
 ProfileProfile   PreferencesPreferences   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Forum index » Science and Technology » Engineering » Mechanics
scissor jack mechanisms
Post new topic   Reply to topic Page 1 of 1 [9 Posts] View previous topic :: View next topic
Author Message
YouGoFirst
science forum beginner


Joined: 13 Jul 2005
Posts: 47

PostPosted: Fri Feb 24, 2006 10:36 pm    Post subject: Re: scissor jack mechanisms Reply with quote

Quote:
This is a scissor jack mechanism. The user sits on the seat (shown at
bottom of mechanism) and pulls themself up using the handles at the
top. The idea is that I can adjust the fulcrum point of the bars and
therefore change the mechanical advantage and weight that I am lifting
(in the range 25% to 75% of my body weight).


My first question is why not use a couple of pulleys and some weights?

With a very simple device you can accomplish the same thing, unless there is
a reason why you want to make is so that you flap like a bird. In that
case, you could easily make it so that you can do that too. If this is for
exercise, you will be better off using pulleys because with them you can
stay on the floor and have constant resistance, versus the variable
resistance that you would have on your system.

If you described what you intend to use your device for, it may help.
Back to top
N:dlzc D:aol T:com (dlzc)
science forum Guru


Joined: 25 Mar 2005
Posts: 2835

PostPosted: Thu Feb 23, 2006 4:21 am    Post subject: Re: scissor jack mechanisms Reply with quote

Dear will_usher:
Quote:

will_usher@yahoo.co.uk> wrote in message
news:1140611944.476762.293160@z14g2000cwz.googlegroups.com...
....
The force on each handle is a vector. Since the
loading on the handle is simple, it vector
representation is not. A significant portion of
loading can be directed from the handle towards
the central pin. So you will need to concentrate
on force components normal to the member's
long axis, and simply realize that the co-linear
force components are there simply to provide the
proper balance. Since you have pinned joints,
those pinned joints will have net torques to
consider (net to zero, but this is only contraining
the problem, and helps). The central pin will have torques
applied, as it will see the manual loads
and the redirected "butt loads" as counter acting
torques.

http://www.nait.org/jit/Articles/reifschneider100405.pdf
.... your mechanism is a version of the "crank-slider mechanism"
The slider is one half of the "butt plate", and it sees one half
the load. The crank is the lower "half" of the member containing
your hand grip. The minimum force applied will be when your hand
delivers force normal to the long axis of this "crank plus
extension" member.

A few more hits on google with:
"four bar mechanism" "crank-slider" site:.edu

I won't be solving it.

David A. Smith
Back to top
tonyp
science forum beginner


Joined: 01 Jun 2005
Posts: 4

PostPosted: Wed Feb 22, 2006 11:14 pm    Post subject: Re: scissor jack mechanisms Reply with quote

<will_usher@yahoo.co.uk> wrote

Quote:
no... those handles are above head height.
The user it pulling them down to lift their own weight.
The flat piece at the bottom is the part that they sit on.
Thats why I'm thinking that when they pull on the handles
they are lifting their own body weight slightly...


Let me jump in here.

Consider a _very_ simple system: a man weighing 600N ties a noose around
his neck, throws the rope over a pulley in the ceiling, and pulls down on
the other end with his arms until his feet are clear of the ground. How
much force are his arms pulling the rope with?

Clearly, 300N. His neck is pulling on the noose with the other 300N :-)

Clearly, too, if it is _another_ man, standing on the ground, who pulls on
the rope, _that_ man would have to exert 600N with his arms to keep the
first man suspended off the ground.

Your machine is much less gruesome, but the same physics applies. The
user's 600N body weight is supported by his arms and his butt. When he just
sits in the seat, it's 600N for his butt, zero for his arms. If, at some
point in the stroke, his arms are pulling on the handles with 300N, then at
that point:

1) His butt is pushing on the seat with 300N, and
2) Moving the handles 1cm closer to the ground will move the seat 1cm
farther from the ground.

A linkage is nothing but a combination of levers. That's nice because a
lever keeps the same force ratio (in a given direction like "up/down") all
through its stroke, because it keeps the same "gear ratio" all through its
stroke.

Does this help?

-- TP
Back to top
will_usher@yahoo.co.uk
science forum beginner


Joined: 21 Feb 2006
Posts: 7

PostPosted: Wed Feb 22, 2006 5:01 pm    Post subject: Re: scissor jack mechanisms Reply with quote

no... those handles are above head height. The user it pulling them
down to lift their own weight. The flat piece at the bottom is the part
that they sit on. Thats why I'm thinking that when they pull on the
handles they are lifting their own body weight slightly... its
different from someone standing on the ground and pulling on the
handles to lift the weight of that person sat on the seat, right?...
the force that they'd have to apply to the handles would be slightly
less.

If you could have a go at this I would be very grateful!
Back to top
will_usher@yahoo.co.uk
science forum beginner


Joined: 21 Feb 2006
Posts: 7

PostPosted: Wed Feb 22, 2006 4:30 pm    Post subject: Re: scissor jack mechanisms Reply with quote

N:dlzc D:aol T:com (dlzc) wrote:
Quote:
Dear will_usher:

will_usher@yahoo.co.uk> wrote in message
news:1140611944.476762.293160@z14g2000cwz.googlegroups.com...

N:dlzc D:aol T:com (dlzc) wrote:
Dear will_usher:

will_usher@yahoo.co.uk> wrote in message
news:1140521176.388396.193610@z14g2000cwz.googlegroups.com...

http://static.flickr.com/34/102559720_59f3f47c73.jpg?v=0

"Lifting oneself by one's bootstraps..."

Could anyone give me any links to basic/in depth
mechanical formulae to calculate this or any insight
into whether this can actually be achieved? (PS I'm
not an engineer so that basics maybe better to
begin with).

It can work, but the system will be largely
indeterminate. Meaning you don't know what the
forces will be, and will definitely be different even
for the same individual through a few minute
routine.

The weight of the individual will be applied
constantly to the central pin, plus/minus
accelerations of the body. Note that downward
force components on the "handles" will also
serve to lift weight off the "foot rest".

Statics will be sufficient to design this. Also
search for "four bar mechanism", which your
apparatus has defined (1: central pin
to outboard pin, 2: outboard pin to foot pin,
3: foot pin to midpoint of foot platform,
4: "slider" from midpoint back to central pin.)

I don't find much in the way of on-line
resources on this. If you think it will be a
product, you may need to hire a
professional for safety concerns.


As i've modelled this on CAD

Escher can be modelled in CAD...

I was thinking of measuring the total
vertical distance moved downwards by
the handle and comparing it to the
total upwards movement by the seat
and by comparing the ratio I could
calculate the force required for the
handle (by using the weight of the
user).

The force on each handle is a vector. Since the loading on the
handle is simple, it vector representation is not. A significant
portion of loading can be directed from the handle towards the
central pin. So you will need to concentrate on force components
normal to the member's long axis, and simply realize that the
co-linear force components are there simply to provide the proper
balance. Since you have pinned joints, those pinned joints will
have net torques to consider (net to zero, but this is only
contraining the problem, and helps). The central pin will have
torques applied, as it will see the manual loads and the
redirected "butt loads" as counter acting torques.

From what you just mentioned I think this method
is inaccurate as with this method you are assuming
that someone else (not the user sat on the seat) is
pulling the handles to lift the weight.

No, I was assuming that the person was standing on the platform,
however. But indeterminate it still is.

I'm assuming I'd have to take into account the
force that is lifting the user created by pulling

Look at the mechanism again. The exercisee will have to push
outwards to lift his/her weight. And will have to restrain an
internally directed force to keep the handles from crushing
his/her ribs/gut. And if one of your hands gets sweaty and
slips... lawsuit.

on the handles in the first place (as you
mentioned).

What if have done is;

1. (I am assuming the user wants to apply
a total force of 600N to the handles) so I
calculated the upwards force lifted the user
off the seat that this would create.

2. I then subtracted this force from the force
needed to lift the body weight and the result
is the force applied to the handles to simulate
600N.

Is this a suitable way of calculating this?

No.

This will take quite a while to set up, and just a little bit to
solve. I'm off to work now. If someone else doesn't set it up
for you, or provide a link to "teach the man to fish", I may take
a stab at it tonight.

David A. Smith

No... the handles are above head height and the user pulls them
downwards, so no crushing of ribs. The flat piece at the bottom is the
seat. Thats why I'm assuming that as you pull down you are slightly
lifting your own weight (i.e. this is different from someone else,
stood on the ground, pulling the handles to lift the user).

I would be very grateful if you could give this a go!
Back to top
N:dlzc D:aol T:com (dlzc)
science forum Guru


Joined: 25 Mar 2005
Posts: 2835

PostPosted: Wed Feb 22, 2006 1:20 pm    Post subject: Re: scissor jack mechanisms Reply with quote

Dear will_usher:

<will_usher@yahoo.co.uk> wrote in message
news:1140611944.476762.293160@z14g2000cwz.googlegroups.com...
Quote:

N:dlzc D:aol T:com (dlzc) wrote:
Dear will_usher:

will_usher@yahoo.co.uk> wrote in message
news:1140521176.388396.193610@z14g2000cwz.googlegroups.com...

http://static.flickr.com/34/102559720_59f3f47c73.jpg?v=0

"Lifting oneself by one's bootstraps..."

Could anyone give me any links to basic/in depth
mechanical formulae to calculate this or any insight
into whether this can actually be achieved? (PS I'm
not an engineer so that basics maybe better to
begin with).

It can work, but the system will be largely
indeterminate. Meaning you don't know what the
forces will be, and will definitely be different even
for the same individual through a few minute
routine.

The weight of the individual will be applied
constantly to the central pin, plus/minus
accelerations of the body. Note that downward
force components on the "handles" will also
serve to lift weight off the "foot rest".

Statics will be sufficient to design this. Also
search for "four bar mechanism", which your
apparatus has defined (1: central pin
to outboard pin, 2: outboard pin to foot pin,
3: foot pin to midpoint of foot platform,
4: "slider" from midpoint back to central pin.)

I don't find much in the way of on-line
resources on this. If you think it will be a
product, you may need to hire a
professional for safety concerns.


As i've modelled this on CAD

Escher can be modelled in CAD...

Quote:
I was thinking of measuring the total
vertical distance moved downwards by
the handle and comparing it to the
total upwards movement by the seat
and by comparing the ratio I could
calculate the force required for the
handle (by using the weight of the
user).

The force on each handle is a vector. Since the loading on the
handle is simple, it vector representation is not. A significant
portion of loading can be directed from the handle towards the
central pin. So you will need to concentrate on force components
normal to the member's long axis, and simply realize that the
co-linear force components are there simply to provide the proper
balance. Since you have pinned joints, those pinned joints will
have net torques to consider (net to zero, but this is only
contraining the problem, and helps). The central pin will have
torques applied, as it will see the manual loads and the
redirected "butt loads" as counter acting torques.

Quote:
From what you just mentioned I think this method
is inaccurate as with this method you are assuming
that someone else (not the user sat on the seat) is
pulling the handles to lift the weight.

No, I was assuming that the person was standing on the platform,
however. But indeterminate it still is.

Quote:
I'm assuming I'd have to take into account the
force that is lifting the user created by pulling

Look at the mechanism again. The exercisee will have to push
outwards to lift his/her weight. And will have to restrain an
internally directed force to keep the handles from crushing
his/her ribs/gut. And if one of your hands gets sweaty and
slips... lawsuit.

Quote:
on the handles in the first place (as you
mentioned).

What if have done is;

1. (I am assuming the user wants to apply
a total force of 600N to the handles) so I
calculated the upwards force lifted the user
off the seat that this would create.

2. I then subtracted this force from the force
needed to lift the body weight and the result
is the force applied to the handles to simulate
600N.

Is this a suitable way of calculating this?

No.

This will take quite a while to set up, and just a little bit to
solve. I'm off to work now. If someone else doesn't set it up
for you, or provide a link to "teach the man to fish", I may take
a stab at it tonight.

David A. Smith
Back to top
will_usher@yahoo.co.uk
science forum beginner


Joined: 21 Feb 2006
Posts: 7

PostPosted: Wed Feb 22, 2006 12:39 pm    Post subject: Re: scissor jack mechanisms Reply with quote

N:dlzc D:aol T:com (dlzc) wrote:
Quote:
Dear will_usher:

will_usher@yahoo.co.uk> wrote in message
news:1140521176.388396.193610@z14g2000cwz.googlegroups.com...

http://static.flickr.com/34/102559720_59f3f47c73.jpg?v=0

"Lifting oneself by one's bootstraps..."

Could anyone give me any links to basic/in depth
mechanical formulae to calculate this or any insight
into whether this can actually be achieved? (PS I'm
not an engineer so that basics maybe better to
begin with).

It can work, but the system will be largely indeterminate.
Meaning you don't know what the forces will be, and will
definitely be different even for the same individual through a
few minute routine.

The weight of the individual will be applied constantly to the
central pin, plus/minus accelerations of the body. Note that
downward force components on the "handles" will also serve to
lift weight off the "foot rest".

Statics will be sufficient to design this. Also search for "four
bar mechanism", which your apparatus has defined (1: central pin
to outboard pin, 2: outboard pin to foot pin, 3: foot pin to
midpoint of foot platform, 4: "slider" from midpoint back to
central pin.)

I don't find much in the way of on-line resources on this. If
you think it will be a product, you may need to hire a
professional for safety concerns.

David A. Smith


As i've modelled this on CAD I was thinking of measuring the total
vertical distance moved downwards by the handle and comparing it to the
total upwards movement by the seat and by comparing the ratio I could
calculate the force required for the handle (by using the weight of the
user).

Quote:
From what you just mentioned I think this method is inaccurate as with
this method you are assuming that someone else (not the user sat on the

seat) is pulling the handles to lift the weight.

I'm assuming I'd have to take into account the force that is lifting
the user created by pulling on the handles in the first place (as you
mentioned).

What if have done is;

1. (I am assuming the user wants to apply a total force of 600N to the
handles) so I calculated the upwards force lifted the user off the seat
that this would create.

2. I then subtracted this force from the force needed to lift the body
weight and the result is the force applied to the handles to simulate
600N.

Is this a suitable way of calculating this?
Back to top
N:dlzc D:aol T:com (dlzc)
science forum Guru


Joined: 25 Mar 2005
Posts: 2835

PostPosted: Wed Feb 22, 2006 3:14 am    Post subject: Re: scissor jack mechanisms Reply with quote

Dear will_usher:

<will_usher@yahoo.co.uk> wrote in message
news:1140521176.388396.193610@z14g2000cwz.googlegroups.com...
Quote:

http://static.flickr.com/34/102559720_59f3f47c73.jpg?v=0

"Lifting oneself by one's bootstraps..."

Quote:
Could anyone give me any links to basic/in depth
mechanical formulae to calculate this or any insight
into whether this can actually be achieved? (PS I'm
not an engineer so that basics maybe better to
begin with).

It can work, but the system will be largely indeterminate.
Meaning you don't know what the forces will be, and will
definitely be different even for the same individual through a
few minute routine.

The weight of the individual will be applied constantly to the
central pin, plus/minus accelerations of the body. Note that
downward force components on the "handles" will also serve to
lift weight off the "foot rest".

Statics will be sufficient to design this. Also search for "four
bar mechanism", which your apparatus has defined (1: central pin
to outboard pin, 2: outboard pin to foot pin, 3: foot pin to
midpoint of foot platform, 4: "slider" from midpoint back to
central pin.)

I don't find much in the way of on-line resources on this. If
you think it will be a product, you may need to hire a
professional for safety concerns.

David A. Smith
Back to top
will_usher@yahoo.co.uk
science forum beginner


Joined: 21 Feb 2006
Posts: 7

PostPosted: Tue Feb 21, 2006 11:26 am    Post subject: scissor jack mechanisms Reply with quote

Hi,

I'm trying to design a device for lifting my own weight, which I've
bascially modelled using 3D CAD, shown here;

http://static.flickr.com/34/102559720_59f3f47c73.jpg?v=0

This is a scissor jack mechanism. The user sits on the seat (shown at
bottom of mechanism) and pulls themself up using the handles at the
top. The idea is that I can adjust the fulcrum point of the bars and
therefore change the mechanical advantage and weight that I am lifting
(in the range 25% to 75% of my body weight).

Could anyone give me any links to basic/in depth mechanical formulae to
calculate this or any insight into whether this can actually be
achieved? (PS I'm not an engineer so that basics maybe better to begin
with).

Your help would be very much appreciated.
Back to top
Google

Back to top
Display posts from previous:   
Post new topic   Reply to topic Page 1 of 1 [9 Posts] View previous topic :: View next topic
The time now is Fri Aug 17, 2018 12:21 am | All times are GMT
Forum index » Science and Technology » Engineering » Mechanics
Jump to:  

Similar Topics
Topic Author Forum Replies Last Post
No new posts mechanisms DVD will_usher@yahoo.co.uk Mechanics 1 Thu Jun 08, 2006 1:57 pm
No new posts Methanol Poisoning; Symptoms and Mechanisms Radium Chem 3 Sat May 27, 2006 5:11 pm
No new posts Jack Sarfatti - Mars CrystalDragonClear Math 4 Sat May 06, 2006 9:16 pm
No new posts Jack Sarfatti's Twelve Step Program to String Theory Addicts Jack Sarfatti Particle 0 Sat Mar 11, 2006 11:38 pm
No new posts SN1 and E1 Mechanisms starting from tert-butyl alcohol? mlc.holder@gmail.com Chem 1 Wed Jan 18, 2006 2:03 am

Copyright © 2004-2005 DeniX Solutions SRL
Other DeniX Solutions sites: Electronics forum |  Medicine forum |  Unix/Linux blog |  Unix/Linux documentation |  Unix/Linux forums  |  send newsletters
 


Powered by phpBB © 2001, 2005 phpBB Group
[ Time: 0.0192s ][ Queries: 20 (0.0021s) ][ GZIP on - Debug on ]