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Jeff Finlayson
science forum Guru Wannabe

Joined: 02 May 2005
Posts: 142

Posted: Fri Jun 16, 2006 3:26 pm    Post subject: Re: Angle Calculation

Jeff Finlayson wrote:
 Quote: eromlignod wrote: Jeff Finlayson wrote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). .... Yes it does. You need to know which length to use to go with the height to determine the angle (or determine the height from the angle). He asked for the angle. You don't need any size dimensions to determine the slip angle. When he machines the wedges he will set the angle on his mill in degrees. The angle can go in either direction...or even at a rotated gradient. The length and height of the block are immaterial. That's all fine. But the design envelop was given. The choke's max angle is 8.88-14 deg depending on which length dimension is used.

That's based on 2 triangular wedges. Tapered pentagonal wedges
will yield a wider range of angles.
hob

Joined: 03 Aug 2005
Posts: 50

Posted: Fri Jun 16, 2006 3:09 am    Post subject: Re: Angle Calculation

<seanlisa@bigpond.net.au> wrote in message
 Quote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself.

rule of thumb for brakes and gears is self-engagement (no slip) at less than
3 degrees.

Mark's Handbook has details on it - brakes section.

>
Nude
science forum beginner

Joined: 15 Jun 2006
Posts: 2

Posted: Thu Jun 15, 2006 7:44 am    Post subject: Re: Angle Calculation

The wedges run along the 400mm length

Jeff Finlayson wrote:
 Quote: seanlisa wrote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself. The wedges run the long dimension (400 mm) of the choke or the short dimension? Unless the blocks are lubed somehow, a low number for the coef. of friction should be 0.1. That's for steel on steel.
Nude
science forum beginner

Joined: 15 Jun 2006
Posts: 2

Posted: Thu Jun 15, 2006 7:38 am    Post subject: Re: Angle Calculation

Yes they will be steel on steel with a machined finish

eromlignod wrote:
 Quote: seanlisa@bigpond.net.au wrote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself. We'll need to know what material the chocks are made from. Are they steel on steel? Don Kansas City
Brian Whatcott
science forum Guru Wannabe

Joined: 09 May 2005
Posts: 267

Posted: Thu Jun 15, 2006 3:51 am    Post subject: Re: Angle Calculation

On 13 Jun 2006 22:07:13 -0700, seanlisa@bigpond.net.au wrote:

 Quote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself.

I liked Don's response, though it must be said he cast his net wide.

I have been chided in the past for offering an angle of 15 degrees as
providing non-loosening grip, from such things as taper pins.
People have mentioned more conservative values, less than 3 degrees if
I recall - but it stuck with me that someone offered the cautionary
note that no taper at all can be considered proof against vibration -
which can move the adjacent surfaces.

Brian Whatcott Altus OK
Jeff Finlayson
science forum Guru Wannabe

Joined: 02 May 2005
Posts: 142

Posted: Wed Jun 14, 2006 7:28 pm    Post subject: Re: Angle Calculation

eromlignod wrote:
 Quote: Jeff Finlayson wrote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). ....

 Quote: Yes it does. You need to know which length to use to go with the height to determine the angle (or determine the height from the angle). He asked for the angle. You don't need any size dimensions to determine the slip angle. When he machines the wedges he will set the angle on his mill in degrees. The angle can go in either direction...or even at a rotated gradient. The length and height of the block are immaterial.

That's all fine. But the design envelop was given. The choke's
max angle is 8.88-14 deg depending on which length dimension is used.
eromlignod

Joined: 02 May 2005
Posts: 68

Posted: Wed Jun 14, 2006 6:04 pm    Post subject: Re: Angle Calculation

Jeff Finlayson wrote:
 Quote: Yes it does! You need to know which length to use to go with the height to determine the angle (or determine the height from the angle).

He asked for the angle. You don't need any size dimensions to
determine the slip angle. When he machines the wedges he will set the
angle on his mill in degrees. The angle can go in either
direction...or even at a rotated gradient. The length and height of
the block are immaterial.

Don
Kansas City
Jeff Finlayson
science forum Guru Wannabe

Joined: 02 May 2005
Posts: 142

Posted: Wed Jun 14, 2006 5:07 pm    Post subject: Re: Angle Calculation

eromlignod wrote:
 Quote: Jeff Finlayson wrote: seanlisa wrote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself. The wedges run the long dimension (400 mm) of the choke or the short dimension? Unless the blocks are lubed somehow, a low number for the coef. of friction should be 0.1. That's for steel on steel. It does not matter which direction the wedge runs, nor do the linear dimensions matter at all. Friction is independent of surface area and only depends on the coefficient of friction between the two surfaces and the normal force applied. In fact, in this case only the angle matters, independent of the weight. If you do a free body diagram, the normal force on the wedge is

Yes it does! You need to know which length to use to go with the
height to determine the angle (or determine the height from the angle).

 Quote: N = w * cos A where A is the wedge angle and w is the weight applied. The force along the slope is F = w * sin A The resisting friction force f is f = u * N = u * w * cos A where u is the static coefficient of friction. The point at which the wedge breaks free is when F > f, so w * sin A > u * w * cos A The weight w cancels out and we're left with u < tan A So the tangent of the wedge angle must be less than the coefficient of friction to prevent sliding. For clean steel on steel, u = 0.8 approximately and the critical angle is A = arctan (0. = 38.7 deg. But this is the critical point where the wedge comes loose. To be safe we must have a smaller angle. Really, to be sure that the chock doesn't slide even if it suffers a shock and breaks the static friction, we should use the coefficient of kinetic friction, so that it still can't slide. So, u = .4 and A = 21.8 deg maximum. If the wedge is in an oily environment, then u = .03 (kinetic) and A = 1.72 deg. max., even if oiled. Don Kansas City
eromlignod

Joined: 02 May 2005
Posts: 68

Posted: Wed Jun 14, 2006 3:47 pm    Post subject: Re: Angle Calculation

Jeff Finlayson wrote:
 Quote: seanlisa wrote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself. The wedges run the long dimension (400 mm) of the choke or the short dimension? Unless the blocks are lubed somehow, a low number for the coef. of friction should be 0.1. That's for steel on steel.

It does not matter which direction the wedge runs, nor do the linear
dimensions matter at all. Friction is independent of surface area and
only depends on the coefficient of friction between the two surfaces
and the normal force applied.

In fact, in this case only the angle matters, independent of the
weight. If you do a free body diagram, the normal force on the wedge
is

N = w * cos A

where A is the wedge angle and w is the weight applied. The force
along the slope is

F = w * sin A

The resisting friction force f is

f = u * N = u * w * cos A

where u is the static coefficient of friction. The point at which the
wedge breaks free is when F > f, so

w * sin A > u * w * cos A

The weight w cancels out and we're left with

u < tan A

So the tangent of the wedge angle must be less than the coefficient of
friction to prevent sliding.

For clean steel on steel, u = 0.8 approximately and the critical angle
is

A = arctan (0. = 38.7 deg.

But this is the critical point where the wedge comes loose. To be safe
we must have a smaller angle. Really, to be sure that the chock
doesn't slide even if it suffers a shock and breaks the static
friction, we should use the coefficient of kinetic friction, so that it
still can't slide.

So, u = .4 and A = 21.8 deg maximum.

If the wedge is in an oily environment, then u = .03 (kinetic) and

A = 1.72 deg. max., even if oiled.

Don
Kansas City
Jeff Finlayson
science forum Guru Wannabe

Joined: 02 May 2005
Posts: 142

Posted: Wed Jun 14, 2006 2:33 pm    Post subject: Re: Angle Calculation

seanlisa wrote:

 Quote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself.

The wedges run the long dimension (400 mm) of the choke or the
short dimension?

Unless the blocks are lubed somehow, a low number for the
coef. of friction should be 0.1. That's for steel on steel.
eromlignod

Joined: 02 May 2005
Posts: 68

Posted: Wed Jun 14, 2006 2:15 pm    Post subject: Re: Angle Calculation

seanlisa@bigpond.net.au wrote:
 Quote: I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself.

We'll need to know what material the chocks are made from. Are they
steel on steel?

Don
Kansas City
seanlisa@bigpond.net.au
science forum beginner

Joined: 14 Jun 2006
Posts: 1

 Posted: Wed Jun 14, 2006 5:07 am    Post subject: Angle Calculation I need help in determining the angle at which slippage will not occur at between two tapered wedges that make up an adjustable chock that will sit under a machine. Each chock is 400 x 250mm. The height need to be 62.5+/-5mm. Each chock will be carrying 48T(metric). Anyone info would be much appreciated as I can not calculate this by myself.

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