Crutch Length

John Webb

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I have a dumb simple question. What effect does positioning the crutch loop farther up or down on the pendulum rod have on the way a clock runs (assuming there is plenty of excess wire length to work with)? I have an old Ingraham mantle clock with a crutch that doesn't look original. Experimenting with this short clock, I don't notice any difference when adjusting the loop up or down, but I'm wondering if the effects would be more critical on a clock with a long pendulum.
 

John Webb

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I have a dumb simple question. What effect does positioning the crutch loop farther up or down on the pendulum rod have on the way a clock runs (assuming there is plenty of excess wire length to work with)? I have an old Ingraham mantle clock with a crutch that doesn't look original. Experimenting with this short clock, I don't notice any difference when adjusting the loop up or down, but I'm wondering if the effects would be more critical on a clock with a long pendulum.
 

Mike Phelan

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I think that is only an issue if the suspension point is above or below the pallet pivot, as on American clocks.
That will mean that the crutch loop sliding on the pendulum rod will move as the crutch length increases, so more friction.
If the suspension is coincident to the pallet pivot, there should be no difference in performance.
JMO
 

shutterbug

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I'm curious if shortening the length would increase the swing of the pendulum. It seems reasonable that it would, and if so, the converse would also prove true.
 

eskmill

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Sorry, it doesn't work that way.

Except for frictional losses, the length of the crutch lever has no effect on the escapement impulse delivered to the pendulum.

Think of the crutch as a lever and apply physics laws to the crutch lever.
 

Scottie-TX

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NOPE. I know what you're thinking. I pondered, myself, some time ago. But, "no" - as Mike counseled and; Regardless of where the point of impulse imparted to the pendulum - regulation is still determined by the EFFECTIVE length of the pendulum beginning at the first point of flexion of the suspension. That's the intersection of the ribbon with the post.
In cases where the pendulum is case-mounted you can benefit by this fact by enabling one to raise or lower the mounting post to achieve the most desirable pendulum position within the case. Perhaps you want it to swing lower or higher in the case. The crutch interface will change but the effective length will not.
 

Ralph

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As Mike mentioned, the optimum hanging point of the pendulum is where the flexure point (or knife edge) is in line with the crutch arbor. Moving from that point will increase sliding friction at the coupling of the crutch to the pendulum rod.

Ralph
 

Scottie-TX

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Moving from that point will increase sliding friction at the coupling of the crutch to the pendulum rod.
RALPH; MIKE; I believe you but I keep reading, reading, and re-reading. There's something here I don't get. As the intersection of the crutch and pendulum becomes further from that plane you described in either direction - above or below - how does friction increase? I just can't get that thru my thick skull and I believe you make an important point I want to understand. You're well aware of my pursuit for efficiency. I'm just missin' something here. What is it?
"sliding on the pendulum rod will more as the crutch "
MIKE elided probably the key word or words between "will" and "more" that may make it understandable. It "what" more?
 
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Ralph

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The way I picture it is that if the pendulum flexure point to the coupling and crutch pivot to coupling are the same length, they are operating in the same arc.

If the lengths are not equal, as they swing through their respective arcs, the mating points of the coupling are sliding in relation to each other.... friction , that proper design could easily eliminate/minimize.

Notwithstanding, that many clocks work fine violating that goal. You'll find precision regulators and most fine clocks.....including Viennas seem to subscribe to this design point.

Cheers, Ralph
 

Scottie-TX

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O.K. Okay! You got through. Long journey eh? Yeah. O.K. I see those two lengths in harmony being desirable. I understand the friction related to deviation. Sort of like when you turn a corner and the wheel on one side travels thru a short arc and the other thru a longer arc. THANKS! Th' dense one finally gets it.
 

Mike Phelan

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Originally posted by Scottie-TX:
Moving from that point will increase sliding friction at the coupling of the crutch to the pendulum rod.
RALPH; MIKE; I believe you but I keep reading, reading, and re-reading. There's something here I don't get. As the intersection of the crutch and pendulum becomes further from that plane you described in either direction - above or below - how does friction increase? I just can't get that thru my thick skull and I believe you make an important point I want to understand. You're well aware of my pursuit for efficiency. I'm just missin' something here. What is it?
"sliding on the pendulum rod will more as the crutch "
MIKE elided probably the key word or words between "will" and "more" that may make it understandable. It "what" more?
Typo corrected! ;) I've started selling them!

If we start with the pendulum coincident with the pallet pivot, there is no sliding; at the other extreme, say the pendulum is higher - much higher. The crutch are would be the same, in theory, so the sliding will be approximately sin (arc) / crutch length.
As we make the suspension nearer to the pivot, the sliding becomes less.
The other thing is that if the crutch arc remains the same, the pendulum arc will lessen as we move the pendulum up, but it is also likely that the crutch arc change; this depends on all sorts of things - how much power, weight of pendulum, escapement type.
 

John Webb

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I thought I was beginning to understand this, now I'm not sure. In a case where the anchor pivot is higher than the pendulum flexure point.....Is it true that if the vertical distance from the the anchor pivot to the pendulum flexture point EQUALS the distance from the pendulum flexure point to the crutch loop, there will not be any sliding/friction? I have a Sessions mantel clock with the anchor pivot offset from the pendulum flexure point by 3/4 inch up, and 1/2 inch right. It is not possible to match the distances because the flat suspension spring is over an inch long. The crutch loop is moving up and down 1/16 inch as the pendulum swings on this clock. Are there any calculations that can be done to position a crutch loop precisely? Is it preferable to bend crutch wires at 90 degree angles so that they come toward the pendulum rod squarely?.....or does it matter at all? John
 

Bill Bassett

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Anyone who has played baseball knows of the "sweet spot" on a bat. This is the point where all of the energy in the bat is transferred to the ball. It is about 8 to 10 inches from the end of the bat. The physics has to do with equalizing angular momentums to the outside and inside of the strike point. If the ball hits too far inside or too far outside if the sweet spot, a batter will feel the impact on his or her hands. Having said all of this, I doubt if the position of the crutch on the pendulum is critical. The major aim is to transfer energy from the escapement to the pendulum.
 

Scottie-TX

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I'll address your last question. I believe you've predicted the answer. "Yes". I believe the angular relationship between the crutch and rod should be a right angle in both vertical and horizontal planes. I believe it reduces angular error and also reduces potential for pendulum wobble and shake. I'd go one step further in the case of crutch loops; That the wire creating the loop is parallel.
 

John Webb

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After thinking this over more, it seems that there will be vertical motion/friction to some degree at the crutch loop in all positions except if the anchor pivot and the pendulum flexion point happened to be on the same point, and in that case, you wouldn't need a loop. The discussion was about vertical height differences, but this would also apply to horizontal differences too, wouldn't it? It just seems like I'm missing something here. I'm looking at 2 different clocks, both with anchor pivots higher than pendulum flexion......One crutch loop is travelling up and down the suspension rod a distance of 1/16 inch, and the other has no noticable movement. Scottie.....I have been bending my crutch rods at 90 degree angles too. It just seems more stable and efficient that way. Whether it actually is or not, I don't know. I need to find a good book with illustractions. All of the beginner books I have start to touch on topics such as this and then they say "....But we won't go into that at this point" John
 

Ralph

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The discussion was about vertical height differences, but this would also apply to horizontal differences too, wouldn't it?
Yes. Ideally the flexure point/knife edge/silk string hanger, etc should be on the same axis as the pallet arbor.

If there was a horizontal offset, a linkage equal to the offset would get you back to the correct geometry, with possible attendant frictional losses......

Keep it on the same axis.

Chhers, Ralph
 
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