Wow. Thanks for all the info. Still wondering if there's any difference
between sheave diameter requirements for applications where the wire rope
bends 90 degrees or more around the sheave and applications where it doesn't
(as in tramming.)
I find snatch blocks very useful for general-purpose rigging, but I guess a
pulley dedicated to tramming doesn't *have* to be a snatch block. But if it
isn't, you have to be sure to bring along some means of attaching it to the
wire rope or the termination hardware once the antenna reaches the tower.
Otherwise, the pulley will slide back down the tram line after the antenna
is unhooked. I prefer taking the pulley off the tram and clipping it to the
tower for later transport down, either via my belt or via the pull rope. But
that's true of the lightweight pulleys I've been using. A heavier, bulkier
model with a large diameter sheave might best be attached to the wire rope
(perhaps with a carabiner through the thimble) so it can be lowered with the
tram.
73, Dick WC1M
> -----Original Message-----
> From: Eric Scace K3NA [mailto:eric@k3na.org]
> Sent: Friday, December 22, 2006 11:17 PM
> To: wc1m@msn.com
> Cc: Tower
> Subject: Re: [TowerTalk] successful tram
>
> Well, actually, K3NA was just reporting from the "Handbook of
> Rigging"
> regarding sheave diameters. To further illuminate the
> subject, I will just quote from the chapter 10, "Blocks,
> Reeving, Sheaves, and Drums":
>
> "If a wire rope is used with inadequately sized sheaves,
> the severe bending stresses imposed on the rope will cause
> wires to break from fatigue, even though actual wear may be
> slight. One of the fastest ways to destroy a [wire] rope is
> to run it over small sheaves. The excessive and repeated
> bending and straightening of the wires leads to premature
> failure from fatigue.
>
> "Small sheaves will also accelerate wear of both rope and
> sheave groove. Since the pressure per unit area of rope on a
> sheave groove for a given load is inversely proportional to
> the size of the sheave, the smaller the sheave diameter, the
> greater the rope pressure per unit area on the groove.
>
> "To determine the unit radial pressure between a rope and
> a sheave, use the formula P = (2 L) / (D d), where:
> P = unit radial pressure, lb/in^2
> L = load on rope, lb
> D = tread diameter of sheave, inches
> d = nominal diameter of rope, inches.
> The allowable unit radial bearing pressures of various
> ropes on different sheave materials are given in Table 10.2."
>
> (Taking the example 100 lb load I gave in earlier messages,
> and using 1/4" diameter wire rope, we can calculate pressure
> for a 8.5" tread diameter and a 2" tread diameter. For the
> big sheave, P = 94 lb/in^2.
> For the small sheave, P = 400 lb/in^2. Remember, the 100 lb
> load of this example is a STATIC load from the dead weight of
> the antenna, with no acceleration force to start the load
> moving up the tram or additional load from a breeze or tag
> lines or bounce or other factors. Table 10.2 provides these
> values for allowable unit radio bearing pressure on various
> sheave materials for regular lay 6x19 wire rope -- the kind
> that WC1M purchased:
> wood: 250 lb/in^2
> cast iron: 480 lb/in^2
> carbon steel: 900 lb/in^2
> chilled cast iron: 1100 lb/in^2
> manganese steel: 2400 lb/in^2
> Continuing with the text...)
>
> "If the unit radial pressure exceeds these maximum values,
> the material from which the groove is manufactured is too
> soft for the operating conditions. Therefore rapid wear of
> the grooves will result.
>
> "The sheave diameter can also influence the rope strength.
> When a wire rope is bent around a sheave, there is a loss of
> effective strength due to the inability of the individual
> strands and wires to adjust themselves entirely to their
> changed position. The rope strength efficiency decreases to
> a marked degree as the sheave diameter is reduced with
> respect to the diameter of the rope.
>
> "Wire rope manufacturers have established standards for
> sheave sizes that should be used with various rope
> constructions (see Table 10.3).
> Always use the maximum possible sheave diameter that the
> lifting equipment will carry."
>
> (Table 10.3 gives recommended and minimum D/d ratio for
> various kinds of wire rope construction. 6x19 Seale wire
> rope has a suggested D/d ratio of 51 and a minimum of 34.
> For d=0.25", the minimum D is 8.5 inch. In this regard the
> Handbook is quite conservative. See also
> https://www.hanessupply.com/page2.html)
>
> "Hard sheave surfaces offer the best bearing surface for
> wire rope, thus prolonging sheave and rope life. If a sheave
> is forged or cast from a material softer than the wire rope,
> the sheave life and the wire rope life will be shortened.
> The sheave will have a tendency to take on the impression of
> the rope, causing scoring and corrugation of the line groove.
>
> "Cast manganese steel sheaves offer the ultimate in sheave
> material.
> This surface actually hardens to the use of wire rope and
> provides greatly extended sheave life and increased service
> time of the wire ropes."
>
> ======
>
> Suppliers: Wire rope snatch blocks are easily located via Google.
> Snatch blocks usually are more expensive and heavier that
> blocks that do not open, prompting one to think: "How
> difficult is it, really, to tread the tram line through the
> block before anchoring to the tower or ground?"
> Sheave data & ordering can be found at, for instance:
> https://www.hanessupply.com/newchanges/sheaves_i.html
> http://catalog.thecrosbygroup.com/maininterface.htm
> and suppliers can be located via Thomas' catalog
> (www.thomasnet.com)
> This equipment is not cheap. But, you buy it only once!
> And it can be a great investment for a club.
>
> For fiberous rope and blocks that work with fiberous rope, I
> (and other
> contesters) have ordered on-line from CMC Rescue Equipment.
> Their prices are reasonable and the equipment is outstanding.
> Again, this is a once-in-a-lifetime purchase of blocks and
> carabiners. I found myself discarding a lot of inexpensive
> hardware store junk after my first experience working with
> CMC materials on a tower -- I had not realized how difficult
> life was with the stuff I was using before. They also have
> climbing gear, including fall protection and work positioning
> harnesses for tower work.
> www.cmcrescue.com
>
> 73,
> -- Eric K3NA
>
>
> on 06 Dec 21 Thu 21:55 Dick Green WC1M said the following:
> > [...snip...]
> >
>
> > Can someone direct me to a good supplier of metal sheave
> snatch blocks
> > suitable for tramming?
> >
> > K3NA says I need at least an 8.5" wheel, but I'd like to revisit the
> > calculations on that. I think they're based on the rope
> making at least a
> > 90-degree turn around the sheave, which doesn't happen in
> the case of the
> > tram. The wire rope does bend around a small portion of the
> wheel's radius,
> > but not much. I suspect a wheel in the 2"-4" range would be
> more than
> > adequate. Comments? What are people out there using for pulleys?
> >
> > 73, Dick WC1M
> >
> >
>
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