The strain is in the direction of the sheave. Typically, a shackle is used
at the top of the pulley, away from the sheave. All surfaces exposed to the
line are smooth, so even a "floating" line under sudden slack won't be
pinched.
In the referenced block, the faces of the sheave are completely covered by
the frame. The space between the sheave and the frame is many times smaller
than the cordage for which it is built. Without a cordage failure (where
all bets are off anyway) there is no place to capture and crimp the cord.
This block is rated at about 200 pounds working load, so, given that it is
a quality product, the user can expect no deformation under normal antenna
loads for 1/4" lines.
The design should keep tension on the cord. This is why most wire antenna
hoist systems are weighted.
The typical hardware store pulley may deform under load, has a lot of space
between the pulley wheel and the frame (slop) and can capture a small line.
Mickey Baker, N4MB
Palm Beach Gardens, FL
*“The servant-leader is servant first… It begins with the natural feeling
that one wants to serve, to serve first. Then conscious choice brings one
to aspire to lead." Robert K. Greenleaf*
On Wed, Mar 10, 2021 at 4:58 PM Richard (Rick) Karlquist <
richard@karlquist.com> wrote:
> I fail to understand why these would be more jam resistant due
> to the mere fact that "the pulley rim is even with the frame of
> the block." That is only true for part of the pulley rim. The
> top quarter or so of the rim (near the "R" in Harken) is exposed
> to a tall frame similar to conventional pulleys. You could also
> have the rope come off the lower part of the pulley and wrap
> around the frame, which cannot happen with conventional blocks.
>
> I don't claim to know anything about pulleys; just trying to
> understand what you are saying.
>
> 73
> Rick N6RK
>
> On 3/10/2021 9:23 AM, Mickey Baker wrote:
> > If you look at the 22mm Harken block, you'll see an example of a design
> > that is difficult to jam - the pully rim is even with the frame of the
> > block. Agreed that they're relatively expensive at $10 or so each, but I
> > have never had a pulley jam. These are designed for sailing where a jam
> > could be a true disaster.
>
> > Mickey Baker, N4MB
>
>
>
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