Re: Re: Re: Re: Re: Re: Re: valve extensions or other remedies


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Posted by Rick Denney on August 18, 2001 at 23:55:15:

In Reply to: Re: Re: Re: Re: Re: Re: valve extensions or other remedies posted by Klaus on August 17, 2001 at 18:36:47:

Klaus, the only way to know for sure is to construct a free-body diagram, which is the one thing we can't do here. So, I'll try to describe what we are both saying in words.

You are correct that the spring and the off-center pressure for what in English is called a force couple, where the finger pressure and the spring pressure are similar. The force couple is resisted by a countering couple provided by the casing. But because the force couple between the finger and the spring is really tiny, the resisting couple is also tiny. In fact, it is even tinier, because the piston is much longer than the offset. So, if you have a 1-inch offset (rather larger than would be the usual case), and you exert one ounce of force, you have a couple of 1 ounce-inch (sorry for the English units). The resisting couple must be the same, but it is spread over, say, a five-inch piston. So the couple is still 1 ounce-inch, but the force is 1/5 ounces (times five inches). This is unlikely to ever create enough force to break the surface tension of the oil, which means it will never cause appreciable wear.

On the other hand, if you press your finger down at an angle, then you don't have a couple, you have a vector that is in a different direction than the piston. So, there will be a force along the piston's axis (the longitudinal component of the vector) and a force at right angles to it (the lateral component). Both add up (geometrically, of course, because this is vector math) to the angle vector representing the force applied by the finger. The spring only resists the longitudinal component, which provides very little resistance, of course. But the lateral component is resisted by the casing, which is effectively immovable. So, if you press on the edge of the button at an angle of, say, 45 degrees (which is what happens when the edge of the button is right in the middle of the contact patch), then you have to use more force to achieve the same one ounce downward force--1.4 times as much. So, your 45-degree vector has a longitudinal one-ounce force and a lateral one-ounce force, which is resisted by the top edge of the piston.

So, when comparing an extreme offset to an angled finger pressure that is perhaps not quite so extreme, and if exerting the same effective downward force, the sideways force on the piston will be perhaps a fifth with the offset what it would be with the angled force.

Rick "who would still rather have a properly pressed offset button than an improperly pressed centered button" Denney


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