Bootstrapping to Save Power - Part 1, Tension Stressing
FLUID POWER - Design Data Sheet 36
What is "Bootstrapping"?
"Bootstrapping" is a technique which can be applied on certain
kinds of applications to save a large amount of input power and to
substantially reduce heat accumulation caused by power waste when
using more conventional techniques.
Bootstrapping methods can be used for holding a constant stress
(tension, torsion, or compression) on material which is traveling
in a continuous and uninterrupted movement through a process or
operation. By conventional techniques, a large amount of power is
wasted by the method used to create the stress in the material a
mechanical brake, hydraulic relief valve, etc. Bootstrapping by the
fluid power method described here can save up to 85% or more of the
stressing power that normally would be lost.
Bootstrapping can also be done with DC electric motor/
generators; but fluid power methods are usually less expensive, use
standard catalog components, and are more controllable. Components
are available for producing very high stress levels over a wide
A bootstrapping system bears some similarity to a regenerative
system in which part of the power is re-circulated to another part
of the system without being dumped. Actually, the only input power
needed for a bootstrap system is enough to keep the material moving
at the desired speed, plus a small amount to make up for
circulation losses in the tensioning part of the circuit where
power is being re-circulated from one hydraulic motor to
Two conventional methods of holding tension on a moving cable
are shown in Figures 1 and 2. An
improved method of doing the same job, using the fluid power
bootstrap principle is shown in Figure 3 on the
opposite side of this sheet.
Tension stressing is featured in this issue. Future issues will
feature torque and compression stressing.
Figure 1. Tensioning With a Brake. This is a
simple system often used on low power applications. Most of the
input power is eventually "burned up" in the brake.
A fixed displacement pump drives a hydraulic motor. A speed
control valve is sometimes added. The hydraulic motor shaft is
coupled to the cable spooling drum, either directly or through
reduction gearing. The cable is looped around an unspooling drum to
which is added a mechanical brake to maintain cable tension. The
brake tension is additive to the tension already existing in the
Obviously, this is a very inefficient
system, and when used on high power applications the brake must be
air or water cooled to dissipate the large amount of wasted
Figure 1. A
mechanical brake is used to hold
tension on a cable as it is unspooled. Tension
is regulated with an adjustable brake spring.
Figure 2. Hydraulic Tensioning. A fixed
displacement pump supplies fluid power to a fixed displacement
hydraulic motor which spools the cable. To provide tension, the
cable is looped around a drum coupled to the shaft of a hydraulic
pump. Flow produced by pump rotation discharges across an
adjustable relief valve, and the tension can be regulated by
adjusting the relief valve.
Like Figure 1, the tensioning is additive to
the normal tension in the cable before it reaches the unspooling
This system is also highly inefficient but does have one
advantage over brake tensioning: the generated heat is carried by
the oil back to the reservoir, and is more easily disposed of by
using a heat exchanger.
Figure 2. A
hydraulic pump and adjustable relief valve
tensioning system is a little more versatile, but
not solve the problem of high energy waste.
Simple Bootstrap System
Most of the power which is wasted in the systems of
Figures 1 and 2 is a result of
holding tension on the cable while it is moving. By using fluid
power for at least this part of the system, most of this power
waste (and oil heating) can be eliminated.
Since bootstrapping applies only to systems where material is
moving in a continuous and uninterrupted flow, in one direction,
hydraulic motors are used instead of cylinders.
The simplest, most economical, and most straightforward way of
designing the system is to separate it into two independent and
unrelated drive systems as follows:
1. Tensioning. As shown in Figure
3, the tensioning part of the design consists of a
variable displacement pump, with pressure compensator, supplying
pressure to a pair of identical hydraulic motors. Motor A should be
plumbed so it will normally run in a CW rotation, and Motor B
should be plumbed to run normally in a CCW rotation. The cable is
strung between these motors on drums of equal diameter. Cable
tension is a result of these two motors pulling in opposite
directions. But since the motors have identical displacement, and
are operating at identical PSI pressure, the system will not move
in either direction.
Figure 3. Two
hydraulic motors, of equal displacement, pull in opposite
directions to keep tension
on the cable. A separate drive source, on the right, supplies
power to overcome the running friction.
2. Motion. Cable movement is provided by a
separate drive coupled to one of the drums. This can be a
completely separate fluid power drive or on some applications could
be an electrical or mechanical drive. It can be a fixed speed
drive, but usually a variable speed is preferred.
As the cable moves through the system, Motor B is driven like a
pump. The oil flow which it produces fills an equal void in Motor
A. Thus, no flow is required from the pump, except a relatively
small amount to make up for slipage the two motors.
Selection of Components
Pump. This should be a variable displacement type
with adjustable pressure compensator. It must be capable of
operating at a PSI pressure sufficient to give the hydraulic motors
the torque they need for the required cable tension. Theoretically,
no flow is required from the pump, but in practice it must supply
sufficient flow for the maximum slippages in itself and the two
motors. Slippage will increase as the units become worn, so an
allowance for increased slippage due to wear should be made.
A piston pump is preferred because it operates with less
slippage and its compensator will adjust over a wider range than
most other types of pumps.
Hydraulic Motors. Two identical fixed
displacement motors should be used. Piston motors give less torque
ripple at low speed than most other types. These motors must be
capable of being over-driven in the reverse direction. Those motors
with built-in speed reduction may not work well.
Note: Each motor, by itself, must produce the
full tensioning torque. Two motors pulling on opposite ends of a
cable produce only the tension of a single motor pulling against a
cable anchored at the opposite end. Both cable drums must be the
same diameter. Calculate motor torque:
Torque (ft-lbs) = Tension
(lbs) × Drum Radius (ft)
RPM required on a cable drum for a desired linear speed on the
cable is calculated with this formula:
RPM = Linear Speed (ft/min)
+ [2π × Drum Radius (ft)]
Relief Valve. As a safety precaution, a relief
valve should be used even with pressure compensated pumps. The
compensator may not give sufficient protection against pressure
spikes on "tight" circuits. (Short connecting lines).
Operation of the System
Cable tension is adjusted with the pressure compensator on the
pump, and will not affect cable speed. Speed may be adjusted with
the external drive and will not affect tension.
Any change in pre-tension, before the
cable enters the drum on Motor B, will affect cable tension. If
this is a critical factor, a mechanism can be added either to read
out the actual tension so it can be re-adjusted by an operator, or
to be linked to the compensator for automatically re-adjusting the
system PSI pressure to keep cable tension constant.
Download a PDF of
Fluid Power Design Data Sheet 36 - Bootstrapping to Save Power -
Part 1, Tension Stressing.
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this