[TowerTalk] HDX-555 raising fixture sheave(s) ?? Part II

[Grant Saviers]

Something to be aware of is the strength of wire rope is reduced when 
wound over a sheave.

The outer strands experience higher stress than the strands along the 
sheave.  This also affects the fatigue life of the cable.  So there are 
standards for diameter of sheave vs diameter of wire rope and tables of 
the strength reduction and fatigue life.
https://www.thecrosbygroup.com/html/en-US/pdf/pgs/380.pdf
https://wirerope.net/jrsg/dratio-strength/
https://www.assemblyspecialty.com/guide-to-wire-rope/technical-information/effective-sheave-size/
http://www.nylatech.com/wp-content/uploads/2013/11/sheave_manual_sm.pdf

Fatigue is not a likely issue for an infrequently used tower hoist 
cable.  The cable you have should be checked for strength over the 
sheave.  From the links, no tower sheave I've seen follows the 
recommendations for the ratio of sheave to wire diameters (crosby: >30 
for 19x7; nylatech: >18). However, the ratios I observed were in the 8 
to 15 range for a strength reduction of around 10 - 20%.  Note the 
correct sheave diameter for the calculation is the "pitch diameter", the 
center of the wound cable, not the sheave od.

Presumably, the original tower engineers did the designs with good 
safety margin, even though outside the standards.

One other sheave source is thecrosbygroup.com/mckissick-sheaves  Most 
rigging suppliers carry crosby.

Grant KZ1W

On 10/15/2020 07:01, Mike & Becca Krzystyniak wrote:
> Greetings again,
> 
> Thanks to all that responded.  Several new leads had the same inventory as
> Grangier and Zoro and others...
> Seems this 3.5" aluminum bearing sheave is a one off and the closest I can
> find is something in a 3" in steel.
> 
> Not being an ME I'm not sure this is a straight forward trade off.
> Assuming the lift weights are the same, i.e. 800 lbs max, is there something
> else to consider?
> 
> Thanks again for all the Sage help.
> 
> 73's  Mike K9MK
> 
> 
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Mike
> & Becca Krzystyniak
> Sent: Wednesday, October 14, 2020 8:29 PM
> To: towertalk at contesting.com
> Subject: [TowerTalk] HDX-555 raising fixture sheave(s) ??
> 
> Greetings,
> 
>     Without going to UST, I have been trying to locate a replacement
> pulley/sheave for my HDX-555 tower.  This is sheave that the raising cable
> uses. There are two slots but there was only one sheave.  Can someone
> confirm whether the tower uses 1 or 2 sheaves, and also a lead on where to
> buy other than UST?
> 
> Spec: 3.5" OD, 1/2 inch thick with 1/2" bolt hole.  It is aluminum with what
> appears to be a pressed in bearing (which has siezed).
> 
> Thanks for any info.
> 
> 73  Mike  K9MK
> 
> 
> 
> 
> 
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