This invention relates to a pneumatic power wrench, in particular
to a pneumatic power wrench having the specific features stated in the preamble
of claim 1.
A general problem concerned with most types of screw joint tightening
tools is to actually accomplish the desired pretension level in all joints, irrespectively
of difference in torque resistance characteristics of the screw joints. A particular
problem of this kind is to avoid undesireable torque overshoot or premature motor
shut-off at tightening of stiff or hard joints, depending on whether the wrench
is of the stalling type or if it is provided with a retardation responsive shut-off
The best way to solve this kind of problem is to reduce the idle or
low-load speed of the motor such that the kinetic energy of the rotating parts as
well as the retardation magnitude is reduced. A lower kinetic energy adds less tightening
torque to the desired target torque level, and lower retardation magnitudes do not
cause any premature shut-off in retardation responsive shut-off mechanisms.
One previously known way to solve the above problems is to provide
the power wrench with a speed governor which reduces the idle speed level of the
motor without impairing the low speed output capacity of the tool. This, however,
is a relatively complicated solution to the problem, since it adds a number of details
and complicates the power tool design.
Another, simpler and commonly used way to solve this kind of problem
is to employ a restriction in the pressure air inlet passage to the motor. This
results in a reduction of the idle speed of the motor and, accordingly, a reduction
in the kinetic energy of the rotating tool parts. However, this solution to the
problem also causes a restriction of the low speed power output of the motor, which
of course is a disadvantage since the full capacity of the tool is not available.
Still another way of reducing the idle speed of a pneumatic power
tool is to restrict the exhaust air outlet flow from the motor. This way is better
than restricting the pressure air inlet flow of the motor, because an outlet flow
restriction is effective in reducing the idle speed of the motor without impairing
the low speed power output of the motor. This is important since it makes it possible
to utilize the full capacity of the motor during the final pretensioning phase of
a screw tightening process.
A prior art publication showing a power wrench with an exhaust flow
restricting valve is a pamphlet and a spare parts list from Gardner-Denver. As can
be appreciated from the exploded view in the spare parts list the "exhaust throttle"
comprises two washers 30, 31, both having banana-shaped apertures. One of these
washers is rotatable relative to the other. At full coincidence between the banana-shaped
apertures there is a low degree of flow restriction, whereas at part-coincidence
only between these apertures gives a higher degree of flow restriction.
This washer type flow restriction, however, has poor flow characteristics,
i.e. the exhaust flow is restricted in an undesired way, and has a power restricting
effect also at low motor speed levels.
A general object of the invention is to provide a pneumatic power
wrench by which the torque overshoot and/or premature shut-off problem is solved
by introduction of an outlet flow restricting means, which without having any negative
effect on the low speed power output and without complicating the power wrench design
effectively reduces the idle speed of the wrench.
A particular object of the invention is to provide a pneumatic power
wrench equipped with an outlet flow restriction of a simple and rugged design, and
which is easy to adjust.
These objects are achieved by providing a pneumatic power wrench as
defined in claim 1.
Other objects and advantages of the invention will appear from the
following specification and claims.
A preferred embodiment of a pneumatic power wrench according to the
invention is described below with reference to the accompanying drawings.
On the drawings
- Fig 1 shows a diagram illustrating the power output characteristics of a power
wrench comprising an outlet flow restriction in comparison with the power output
characteristics of a tool having an unrestricted pneumatic motor and a tool having
an inlet flow restriction.
- Fig. 2 shows a longitudinal section through a power wrench according to the
invention, including the outlet flow restricting means.
- Fig. 3 shows, on a larger scale, a section through the handle of the power wrench
and illustrates the outlet flow restricting means.
In the diagram in Fig. 1, the output torque T as well as the output
power P of an outlet flow restricted motor are illustrated as functions of rotation
speed n. For the purpose of comparison, the diagram also illustrates the output
characteristics of an unrestricted tool and a tool having an inlet flow restriction.
The torque/speed characteristic for an unrestricted tool is illustrated
as a straight continuous line Ta, and the power/speed characteristic is illustrated
by the continuous curve line Pa. The idle or unloaded speed of the unrestricted
tool is n(1).
The general aim of the invention is to reduce the idle speed or unloaded
speed by 20 - 25%. This is represented by the point n(2). By employing an inlet
type of flow restriction, as commonly used, there is obtained a substantially lowered
torque/speed characteristic, as illustrated by the dash dotted line Tb. Compared
to the power/speed characteristic Pa of an unrestricted tool, the power/speed characteristic
Pb of the inlet restricted tool is substantially reduced too, also in the low speed
Accomplishing the same idle speed reduction as by the inlet flow restriction,
to point n(2), the outlet flow restriction causes a substantially less torque reduction,
as illustrated by the dash line Tc. Typically, an outlet restricted tool provides
a much smaller torque reduction than an inlet restricted tool, especially in the
low speed range. The same goes for the reduction in output power compared to an
unrestricted tool. The power /speed characteristic for an outlet restricted tool
is illustrated by the dash line Pc.
One important group of tools to be provided with an outlet flow restricted
motor is pneumatic power wrenches having an hydraulic impulse clutch, in particular
an impulse clutch of the type without a shunt connection between the pressure chamber
compartments. In this type of tools, the first delivered torque impulse tends to
have a significantly higher amount of energy due to a higher speed. This means that
when tightening a hard screw joint where the torque resistance starts very abruptly,
the energy of the first impulse is very high and can easily overtighten the screw
joint. Moreover, if this type of tool is provided with a retardation responsive
shut-off mechanism, the very first impulse could also cause a premature shut-off
of the power supply to the motor.
Looking now at the embodiments of the invention, Fig. 2 shows a section
through a pistol type power wrench comprising a housing 10, a rotation motor 11,
a power transmission in the form of a hydraulic torque impulse clutch 12 and an
output shaft 14. The impulse clutch 12 is provided with an automatic shut-off means
comprising a retardation responsive trigger mechanism 13a mounted on the impulse
clutch and a pressure air inlet shut-off valve 13b located at the rear end of the
housing 10. The shut-off valve 13b is connected to the trigger mechanism 13a via
a push rod 13c which extends axially through the motor 11.
The shut-off mechanism is previously described in US Patent No. 5,082,066
granted to applicant.
Moreover, the housing 10 is formed with a pistol type handle 15 which
comprises a pressure air inlet passage 16 including an air line connection tube
17, threadingly mounted at the lower end of the handle 15, and a throttle valve
18. The latter comprises a tiltable valve element 19 arranged to sealingly cooperate
with a valve seat seal ring 20 and to be operated by a push button 21. A spring
22 takes support against the inner end of the connection tube 17 and biasses the
valve element 19 toward closed position.
In parallel with the air inlet passage 16, there is an exhaust air
outlet passage 23 which via a flow restricting means 24 and an outlet deflector
25 communicates with the atmosphere. The outlet deflector 25 surrounds an outer
portion of the connection tube 11 and is rotatable to enable adjustment of the outlet
Attached to the lower part of the handle, there is an end piece 27
through which extends an axial bore 28 and a threaded valve spindle 29. The thread
on the latter engages a threaded sleeve 30 rigidly secured in the end piece 27,
and by means of an internal screw bit grip 31 in the spindle 29, the latter is rotatble
to adjust its axial position in relation to the end piece 27. On the inner end of
the spindle 29, there is supported a valve element 32 which is formed with a conical
portion 33 for flow restricting cooperation with an annular seat 34 formed by the
inner end of the end piece 27. A lock ring 35 is mounted in a circumferential groove
36 on the spindle 29 to form an axial lock means for the valve element 32. The valve
element 32 is rigidly secured to the spindle 29 between a shoulder 39 on the spindle
29 and the lock ring 35 and the flow restricting opening is variable by adjusting
the axial position of the spindle 29.
In operation of the power tool, the valve spindle 29 is set to accomplish
a flow restricting passage between the valve element 32 and the seat 34 that is
adequate in relation to the desired idle speed. The setting of the restriction valve
24 is adjusted to adapt the output characteristics of the tool to the actual screw
joint characteristics. The desired flow restriction is set by rotating the spindle
29 in either direction.