This invention relates to a multi-core cable intended for communicating
electric power and electrical signals to and from a hand held power nutrunner.
In particular, the invention concerns a multi-core cable of the type
having two or more sections each with a geometric centre and extending in parallel
with each other such that in any cross section of the cable the geometric centres
are disposed on a straight line.
In prior art, electric communication with hand held power nutrunners
is accomplished via cables with the electric conductors arranged in concentrically
disposed cores, i.e. cables with a substantially circular cross section.
One drawback inherent in cables of this known type is that, although
they are universally flexible, they tend to be rather stiff, because the high number
of conductive cores causes a large outer diameter of the cable and, accordingly,
a large radius from the centre of the cable to the outermost located cores.
This causes not only a stiffer cable and a more awkward handling
of the power nutrunner, but results in high tension forces and large relative displacement
of the outermost cores at bending of the cable. This results in turn in a shorter
service life of the cable since frictional wear and the risk for breakage of the
outer cores are high.
Another problem concerning prior art concentric cables refers to
the electric distortion on the signals communicated from the nutrunner. This is
caused by the electromagnetic field existing around the power supplying cores
connecting the nutrunner motor to a power source, and since the signal and power
supplying cores are located very closely to each other the electromagnetic influence
on the signals is inevitable.
A solution to the above mentioned problems is obtained by using a
flat type of cable wherein a better separation of the power and signal communicating
cores may be obtained as well as a shorter distance to the cable centre in one
direction for the outermost cores. The latter feature is advantageous since it
causes less tension and displacement of the outermost cores at bending of the
cable in the direction of the small dimension of the cable. The bending force in
that direction is substantially lower as well compared to a prior art concentric
type of cable.
However, using such a flat type of cable with two or more core sections
located in parallel brings another problem to which this invention is a solution,
namely how to reduce the bending forces as well as the core tensions caused in
the direction of the large dimension of the cable. The large dimension of such
a flat type of cable is much larger that the outer diameter of a concentric type
of cable which means that the flat type of cable is almost completely stiff in
that direction. This means that such a cable would make the handling of the nutrunner
A further problem inherent in prior art cables of circular outer
shape refers to the difficulty to discover whether the cable has been unintentionally
twisted during use of the nutrunner. Such twisting of the cable easily leads to
tangling of the cable which in turn might cause damage to the cable itself as well
as impairment of the nutrunner handling. This problem is solved by using a flat
type of cable, twisting of which is easy to observe.
The above problems are solved by the invention as it is defined in
Preferred embodiments of the invention are below described in detail
with reference to the accompanying drawings, on which
Fig 1 shows a power nutrunner connected to a control and monitoring
unit by means of a cable according to the invention.
Fig 2 shows the rear part of a power nutrunner to which a cable according
to another embodiment of the invention is connected.
Fig 3 is a cross section of the cable at III-III in Fig 2.
The device shown in Fig 1 comprises an electric power nutrunner 10
having a handle 11 for manual support of the nutrunner 10 and a multi connector
jack 12 interconnected with a mating multi connector plug 13 mounted at the end
of a cable 15.
Via the cable 15 the nutrunner 10 is coupled to a unit 14 comprising
electronic control and monitoring equipment by which the operation of the nutrunner
is governed. This equipment comprises power supply means, tightening process controlling
and monitoring means, means for data storing and documentation etc.
The cable 15 is of a flat type comprising three parallel sections
16, 17 and 18. Each of these sections has a geometric centre 20, 21, and 22 respectively,
and all three of these geometric centres 20, 21, 22 are disposed on a straight
line 24. This straight line disposition of the section centres 20, 21, 22 is maintained
throughout the length of the cable 15.
One of the cable section 16 comprises a number of cores for communicating
electric power to the nutrunner 10.
Another section 18 comprises a number of signal communicating cores
coupled to signal producing means like torque transducer, angle encoder, temperature
sensor etc. in the nutrunner 10.
A third section 17, situated between the two other sections 16, 18
does not comprise any electric conductors at all, but includes a cable support
line by which the other two sections are releaved from occuring tension forces.
This electrically inert section 17 also serves as a distance means
for separating the power supplying cores of section 16 from the signal communicating
cores of section 18, thereby reducing considerably the electromagnetic distortions
on the signals transmitted from the nutrunner 10 to the control and monitoring
equipment coupled to the nutrunner 10.
All three sections 16, 17, 18 are firmly kept in the flat cable type
disposition by a synthetic resin moulding 25, such that the large transverse dimension
b is about three times the small dimension a. See Fig 3.
As illustrated in Fig 1, the cable 15 comprises a flex zone A located
adjacent the nutrunner 10, in which zone the cable 15 is preformed to a 180° twisted
shape. This is accomplished by heat treatment of the cable in a specially designed
fixture, wherein the synthetic resin moulding 25 adopts a twisted shape without
changing the relative positions of the core sections 16, 17, 18. Accordingly,
the geometric centres 20, 21, 22 of these sections are maintained on the straight
The flex zone A forms just a minor part of the total length of the
cable, which means that the rest of the cable, which forms a second portion B,
has a straight nontwisted preforming. This makes it possible to check visually
the cable for any undesirable twisting that might cause kinks and damage to the
cable itself as well as an impaired handling of the nutrunner.
In, for instance, assembly line use of electric nutrunners a common
problem is that the cable gets unintentionally twisted due to repeated half way
turns each time the operator picks up the tool and returns it to a rest position.
So after several operation cycles the cable may have been undesireably twisted
shape and, hence, the nutrunner handling impaired.
In one example successfully used in practice, the cable has a total
length of 5,0 m and comprises a flex zone of 0,6 m adjacent the nutrunner. The
flex zone has a 180° twist angle and offers a comfortable handling of the tool.
By the introduction of the flex zone A in accordance with the invention,
the flat type of multi-core cable has been made universally flexible for ensuring
a comfortable handling of the nutrunner. In other words, the invention has made
it possible to use a flat type of cable for this purpose, which in turn has made
it possible to improve not only the service life of a multi-core cable for hand
held power nutrunners but to obtain more reliable and less distorted signals from
By the invention, it has been possible to use a type of cable where
the safety against short circuiting between the power supply cores and the signal
transmitting cores is substantially improved as well. This is an important feature
for protecting equipment as well as personell against hazardous voltage.
It is to be noted though that the invention is not limited to the
above described example, but can be varied within the scope of the claims. For
example, the number of parallel core sections is not limited to three, and the
shape of the flex zone A could have any twist angle from about 180° and upwards.
The above described embodiment including a 180° twist is an example of a well
operating flex zone.
At repeated bending of a flat type cable a certain angle a certain
length of the cable has to be involved in the bending movement to avoid fatigue
stresses in the cable, which length is determined by the endurability to bending
of the cable, i.e. the permissible minimum radius of curvature.
To obtain a universal bending ability of the cable a portion of the
cable is preformed in a twisted shape to form a flex zone A. Bending of the cable
in the flex zone A in any direction means that the actual bending takes place
only in those portions of the cable in which the weakest section is disposed in
the bending direction. As a matter of fact, there is only a limited portion C of
the cable per half twisting turn that has the weakest section in any bending direction.
In Fig 2 the weak portion C is illustrated on that part of the cable
which is the weakest section at bending in directions illustrated by the arrows.
Accordingly, there is only a fraction x % of the flex zone length
that forms the weakest portion in any randomly chosen bending direction. This weak
portion C has to have a sufficient length 1 such that the limit for the bending
ability for the cable is not exceeded at bending over a larger angle. To achieve
this, the length L of the flex zone is: 100x x 1. Depending
on the relationship between the width b and thickness a of the cable,
the weak portion C of the flex zone A varies in length between 10% and 30%, the
greater the relationship between width b and thickness a the smaller
portion of the flex zone A is formed by the weak portion C for a certain pitch
of the twisted shape.
Depending on the bending direction of the tool the weak portion C
of the flex zone A will be located at different distances from the tool. In a case
where the tool is articulated in the direction in which the cable section closest
to the tool has its stiffest characteristic, and the cable flex zone A has its
minimum acceptable twisting angle of 180°, the cable will be bent in the very centre
of the flex zone. Accordingly, this particular bending direction makes the cable
bend at its weakest portion C which is located at a distance from the tool equal
to half the length of the flex zone A. In certain cases, this distance can be
too large to obtain a comfortable handling of the tool. This single point deflection
of the cable may also turn out to be uncomfortable for the operator and unfavourable
for the service life of the cable. To distribute more evenly the bending movement
of the cable, the twisted shape of the flex zone may comprise several full or
half turns to accomplish more weak portions in each and every bending direction.
To ensure comfortable handling of the tool, the length of the flex
zone A may not be too long. The suitable length of the flex zone A is 1/2 - 3 times
the largest dimension of the tool, or 1 m at the most.