The Problem
The savings and benefits of automatically applying lubricant during cable pulling
depend on installation specifics. A contractor pulling a few hundred feet of building wire
into small conduit will have quite different needs from a utility crew pulling several
thousand feet of underground distribution cable daily. Yet in both cases, some level of
automation in the lubrication operation may be cost justified.
Automated lubrication can:
- Save time and manpower, usually by eliminating the need to dedicate a person to applying
lubricant by hand.
- Save optimal lubricant quantity, that is, not using too much or too little lubricant.
- Provide thorough and consistent lubrication for lowest cable pulling tension.
How Much
We first need to know how much lubricant to use when pulling a cable. The equation
below will tell us.
Q = K x L x D
Where
Q = Quantity of lubricant in gals (liters)
L = Length of conduit in feet (meters)
D = ID of the conduit in inches (mm)
K = Constant of 1.5 x 10-3 (English) or 7.3 x 10 -4 (metric)
This equation determines the amount of lubricant needed to completely coat the interior
wall of a conduit with a lubricant film of 0.009 inches (0.2mm) thickness. The equation
calculates lubricant quantities consistent with those used by experienced cable pulling
crews.
How Much/How Fast
If we’re going to pump the lubricant to the conduit entrance, we’ll need the
previous equation converted to a lubrication rate calculation:
R = K x D x S
Where
R = Lubrication rate in gals/min (liters/min)
S = Pulling speed in feet/min (meters/min)
At typical pulling speeds of 10 to 60 ft/min (3 to 18 m/min) and duct sizes of 2 to 6
inches (50 to 150 mm), the rate calculation provides expected lubricant demand from a low
of 0.03 gal/min to a high of 0.5 gal/min (0.1 to 1.9 l/min). These are low volume flow
rates that can be produced by a number of low-power pumps.
But first, what about smaller and/or less frequent pulls, where automatic pumping is
not practical. Is there any better way to apply pulling lubricant than dipping into the
lube bucket and applying it by hand?
Pump Selection
When choosing pumping systems for applying pulling lubricant, a pump should be selected
that can handle both liquid and gel lubes at the needed flow rates. It is a mistake to
compromise lubricant quality based on limitations in a pump’s capability. The most
important characteristics of pulling lubricant are that it is fully compatible with cable
jackets and that it produces low friction in a broad range of field conditions, regardless
of how it is applied.
A pump should not shear the lube or build pressure in the outlet hose when flow is
reduced with a restriction valve. Pressure pots low-ratio piston pumps and air-operated
diaphragm pumps all meet these criteria.
High shear pumps that run at a high constant speed (rotary vane, gear) are usually not
appropriate for field lubrication. The continuous shear from the blades causes
deterioration in lubricant performance and builds high pressure in the feed lines.
The diameter and length of the outlet hose on a pump are also important. The hose and
any other line restrictions should be large enough to support the desired flow rates.
The usual question is whether a pump has enough draw to pull the liquid into the pump.
One of the pump types we recommend, a "pressure pot", has all the liquid
"within" the pump, so priming, draw, and cavitation are not concerns.
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