Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

[Phys-L] Bernard is very par annoyed. Was: Re: Differential screw, was : Re: Chain Hoist Operation



Has anyone else received an e-mail I evidently wrote some time ago?
I've had at least four replies to ones I thought I'd mailed (not from
members of this list), one 18 months ago. That's not all. If I don't
disconnect the ethernet cable w/ in about 10 min of "going on line", my
mail app. becomes corrupted. First scrolling requires continuous mouse
clicking instead of just holding it down; then using the return key
fails requiring instead mouse clicks for auto. selected buttons; finally
this fails. In most cases this makes further use of the app.
impossible, and I must force quit.

Anyone else out there using Netscape 7.2 w/ Panther?

bc, working off line.

p.s. I thought I'd sent the below to the list, not "privately".
p.p.s. Panzers to BW -- I'd never heard of a rigging screw
more: this and another msg. I'd saved but didn't appear in the drafts
folder. When I quit, this appeared with the question did I want to send.

Brian Whatcott wrote:

I am unsure why bc's question popped up in my reader after so long,
but for my own amusement I offer this:
A rigging screw is one heterodox example of a differential screw.
Here the screws are usually of the same pitch, one left hand and the
other right hand, so that turning the body of the screw causes both
screws to enter the body of the rigging screw, or exit - together.
The overall effect is to provide a length change of two screw
pitches per revolution.

A rigging screw designed with two screws of the same chirality,
but with different pitch, would be a more understandable example of
differential action.
Here, one screw departs one pitch, while the other enters the body
by one pitch. The overall length change is the difference of the two
pitches per revolution.

Brian W

At 08:22 PM 6/20/2003, you wrote:


pse. follow up with the differential screw, as found in optical stages
(e.g. interferometers)

bc

John S. Denker wrote:



On 06/20/2003 10:54 AM, Tony Wayne wrote:


Can anyone describe how a manual chain hoist works? I've looked
across the internet and have only found products and demonstrations
utilizing a chain hoist.


It's a differential pulley. Apparently invented by T.A. Weston
some time before 1875.
http://www.yale.com/historypop1875.htm

From a distance it looks like an ordinary block-and-tackle.

The trick is that on one block (let's call it the
upper block) rather than having two free-running
sheaves, the two sheaves are locked to each other,
typically made from a single piece of metal. Also,
one of these two sheaves is slightly smaller than
the other.

There are differential pulleys that use rope or
cable, but in the case of a chain, the sheaves
have pockets to grip the links of the chain,
ensuring zero slip.

In any case, the two sheaves on the upper block
have slightly different diameters.

The device is rigged with an endless chain, forming
two loops. One loop threads the lower block, while
the other loop is just free. In operation, one loop
gets bigger while the other gets smaller.

When you rotate the sheave of the upper block, it
changes the size of the loops in proportion to
the _difference_ in radii of the two sheaves on the
upper block. That's because it is taking up chain
with one sheave while it is letting out chain with
the other.

By making the radii nearly equal, you can get
arbitrarily large mechanical advantage (for
continuous rope or cable). For chains, the
minimum differential is one link, so the mechanical
advantage scales like the number of links that
fit around a sheave (plus a factor of two for the
conventional pulley effect).







Brian Whatcott Altus OK Eureka!