Selecting the right Screwdrivers and Nutrunners
One
of the first things to consider when selecting
a new screwdriver or nutrunner for a particular
application is the type of clutch that
is best suited for the application. The
following information describes the functional
arrangement of each type of clutch and
the types of applications for which each
is best suited.
Direct
Drive Clutches
Direct
connection between the motor and the driving
bit. Fastener is driven until the motor
stalls.
This
is the simplest and least expensive of
all drives. Low maintanence, diaphram-type
pressure regulators can be used to adjust
the stall torque. Reducing the stall torque
in this type of tool also reduces the
operating speed. Motor wear, fluctuations
in pressure, and faulty lubrication are
all factors that will cause variation
in delivered torque. "Kick"
can also be fatiguing to operators.
Positive
Clutches
Motor
and driving bit are connected through
one clutch, the jaws of which have sloping
faces and are normally held apart by a
light spring. This clutch provides a stationary
bit while locating on a fastener even
when the motor is running. When applied
to a fastener, the operator's axial pressure
engages the jaws. Direct drive results
until torque buildup is sufficient to
cause jaws to cam out of engagement against
operator pressure. Jaws ratchet, causing
further torque buildup until the operator
stops the tool or removes it from the
fastener.
With
a positive clutch, the operator's technique
can cause considerable variations in delivered
torque. Frequently used to drive screws
into wood and similar materials where
torque requirements may vary due to knots,
soft spots, etc. Not recommended where
excessive axial pressure on fastener may
damage assembly. Axial impact action of
ratcheting jaws may also cause damage
to some assemblies.
Adjustable
Ratcheting Clutches
Motor
and driving bit are connected through
two clutches. One has 90 degree jaws held
apart by a light spring. This clutch provides
a stationary bit while locating on a fastener
even when the motor is running. When applied
to a fastener, the operator's axial pressure
engages the jaws. The second clutch consists
of two jaws with sloping faces held together
by the compressive force of a heavy spring.
This force can be adjusted by means of
a nut. When torque is sufficient to cause
the jaws of the second clutch to cam apart
against the force of the spring, jaws
disengage and re-engage repeatedly. This
ratcheting action causes further torque
buildup until the operator stops tool
or removes it from the fastener.
A
good general purpose clutch for applications
where close torque control is not required.
Adjustment should be set so desired torque
is achieved shortly after jaws begin to
ratchet. Sound and feel of ratcheting
signals the operator to stop or remove
the tool from the fastener. As the operator's
reflexes slow, due to fatigue or distraction,
torque can become excessive. Noise and
vibration from ratcheting torque can present
problems.
"One
Shot" Clutches
Motor
and driving bit are connected through
two clutches. One has 90 degree jaws held
apart by a light spring. This clutch provides
a stationary bit while locating on a fastener
even when the motor is running. When applied
to a fastener, the operator's axial pressure
engages the jaws. The second clutch consists
of a pair of jaws with precision machined
pockets. Hardened balls rest in the pockets
and are clamped between the jaws by a
heavy spring. Torque setting depends upon
compressive force in the spring, which
is adjustable. When desired torque is
reached the balls tend to roll out of
thier pockets, forcing the jaws apart.
This action separates the 90 degree jaws
and completely disconnects the motor from
the bit. The motor free-wheels and no
further torque is applied to the fastener.
Excellent
for practically all applications, especially
torque-critical jobs such as driving fasteners
into soft materials or clamping brittle
materials. Instantaneous clutch action
is quiet and vibrationless. Clutch maintanence
is minimal due to the absence of ratcheting.
Not recommended where torque requirements
from fastener to fastener are not uniform
(due to mis-alignment of parts, etc.)
or for rare applications where torque
peaks higher than final torque is encountered
during self-tapping or thread forming.
Capacity
In
determining the size of the tool required,
consideration must be given not only to
the size of the screw to be driven and
the final torque required, but also to
the nature of the job. The maximum capacity
of a screwdriver or nutrunner varies for
each type of application.
The
torque imparted to a screw or nut can
come from three sources: (1) From air
pressure against the rotor blades, (2)
From flywheel effect of the motor and
other rotating parts, and (3) from clutch
jaw impact of ratcheting clutches. Heavy
turining resistance on the run-down reduces
speed and consequently reduces torque
available from flywheel effect and jaw
impact. Resilient assemblies absorb both
flywheel energy and jaw impact and thereby
reduce torque produced by these two factors.
The
maximum torque a given tool can impart
to a screw or nut is greatest when driving
"free-running--sudden stop"
screws or nuts used to fasten non-yeilding
materials. Here the resistance to turning
is low throughout the run-down and the
tool runs at close to it's free speed.
Maximum possible torque is imparted from
each of the three sources mentioned above
when the screw or nut seats against a
solid stop.
Normally,
a particular tool will deliver lower torques
on a "soft pull up" job than
any of the other types of jobs since the
heavy resistance to turning reduces the
amount of torque obtained from the flywheel
effect. On other types of jobs illustrated
at the right, a tool will generally deliver
slightly more torque than the maximum
"soft pull up" rating.
For
example, a "soft pull up" condition
would exist in the case of a sheet metal
screw if the screw is head seated before
the hole in the sheet was expanded to
the maximum root diameter of the screw,
since resistance to turning would remain
high throughout the entire run-down. If,
however, the screw were longer and considerable
run-down occurred after the hole in the
sheet was expanded to maximum diameter,
the tool could be expected to pick up
speed due to the decrease in resistance
and the energy available from the flywheel
effect and impacting would increase. Thus
the job would change from a nominal "soft
pull up" condition to something in
between that condition and "free
running--sudden stop".
Impacting
is not a factor with "One Shot"
and direct drive tools. Flywheel effect
causes a noticable variation in torque
delivered by direct drive tools when job
conditions are not uniform. This variation
is less pronounced in "One Shot"
tools.
Reversibility
Non-reversible
models recommended for all normal assembly
work. Reversible models are suggested
for all disassembly work and for assembly
where screws must be frequently removed
to cross threads or defective threads.
Speeds
The
"free speed" of a screwdriver
is the rpm of the drive spindle with no
load, 90 lbs. of air pressure, and with
the speed regulator wide open. Different
"free speeds" are obtained by
using different gear ratios between the
motor and the drive spindle. The actual
speed of any tool can be reduced if desired
by using the speed regulator to adjust
the rate of air flow to the motor.
With
Direct Drive
select a tool with
a "free speed" whose maximum
stall torque is equal to, or greater than,
the torque required by the heaviest screw
to be driven. Then use the speed regulator
to reduce the speed and power the tool
to give exact torque required.
With
Adjustable Ratcheting Clutch
high "free speed" usually gives
greater clutch jaw impacting effect, and
will drive free-running and light pull-up
screws tighter. Reduce speed with the
speed regulator when less impacting effect
is desired. Tools with slow "free-speed"
can deliver greater torque on long, heavy
pull-up jobs.
With
"One-Shot" Clutches,
the degree of torque accuracy generally
increases at lower speeds. The lowest
speed "One-Shot" tools are practically
insensitive to variations in job conditions.
A certain amount of torque variation should
be expected from higher speed tools but
thier accuracy will still be adequate
for most production jobs.
Such
variations will be most pronounded on
jobs approaching "free running--sudden
stop" conditions. When the allowable
torque range specified for a fastener
is extremely narrow, the lowest speed
tool having sufficient torque capacity
is recommended.
Handles
In
most instances a straight handle is used
for driving verticle screws, and an offset
or side handle for driving horizontal
screws. However, the two governing factors
are the position of the operator's forearm
and the position of the screw. If the
screw and the operator's forearm lie parallel
to each other, use an offset or side handle.
If they are perpendicular, use a straight
handle.
Straight
handle with overhead suspension balancer
is a popular arrangement for bench assembly
work where parts can be arranged so that
screws can be installed vertically. Offset
or side handles are better for resisting
torque reaction and recommended for heavier
work, particularly for slow speed direct
drive tools.
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