I rig mine so the rope pulling the antenna up the tower is actually pulling
the pulley and the antenna is hanging from the pulley by a separate rope.
And on another comment about deadends. At the tower end of the tram wire I
use a deadend and the pulley just rides right up over it, something that
wouldn't work if I used cable clamps up there.
David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of Dick Green WC1M
> Sent: Saturday, December 23, 2006 05:13
> To: firstname.lastname@example.org
> Cc: 'Tower'
> Subject: Re: [TowerTalk] successful tram
> 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
> (as in tramming.)
> I find snatch blocks very useful for general-purpose rigging, but I guess
> pulley dedicated to tramming doesn't *have* to be a snatch block. But if
> isn't, you have to be sure to bring along some means of attaching it to
> 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
> tower for later transport down, either via my belt or via the pull rope.
> 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
> 73, Dick WC1M
> > -----Original Message-----
> > From: Eric Scace K3NA [mailto:email@example.com]
> > Sent: Friday, December 22, 2006 11:17 PM
> > To: firstname.lastname@example.org
> > 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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