The invention relates to sewing machines and more particularly to an improved needle bar driving system with decreased vibration and better balance.
In conventional sewing machines designed as suggested by U.S. Pat. No. 2,128,120 having their needle bar drive designed as a crank drive, a counterbalance weight firmly secured to the armshaft of the machine is provided rotating in opposition to the crankpin. The counterweight is selected to equilibrate the rotating masses, namely the crank arm with the crank pin and a portion of the needle bar link, and partly the vertical first order inertia forces which are caused by the needle and a portion of the needle bar link. This partial compensation, however, is a compromise justifiable only for low-speed to medium speed sewing machines, since along with a partial compensation for the vertical inertia forces, a horizontal component develops, so that a rotary inertia force vector is produced varying in magnitude. In high speed sewing machines, such non-compensated inertia forces cause strong vibrations of the machine housing adversely affecting the operation. These vibrations generate high noise.
Several designs like U.S. Pat. Nos. 4,539,922 and 3,040,682 have been suggested to eliminate or decrease the problems outlined above, however, these solutions have proven either less efficient in decreasing vibration, or too complex and expensive in design. While they decreased one or more components of the vibration, they have also caused a further source of unbalance.
Therefore, up to the present, most of existing and commercially available sewing machines use this more than 74 years old design having an asymmetric counterbalance weight connected to and rotating with the armshaft.
There are sewing machines, like quilting machines, where the machine is arranged in a rigid, substantially C-shaped housing with a rear vertical column and respective upper and lower horizontal arms which hold the driving system of the machine, wherein the length of the arms must be long to enable the machine to work with textiles of substantial width. If the needle bar assembly arranged in the forward end portion of the upper arm causes vibration, the stability can be provided only by providing a massive, heavy and robust housing that resists such vibration forces and does not let the needle to vibrate in addition to its reciprocating movement. The problems get more acute when there is a need for high speed operation with increased accuracy.
There is a long-existing need to a better-balanced needle bar driving system, which if can be created, can reduce noise, increase accuracy, enables higher speed operation and last, but not least enables a lighter design for the whole sewing machine. The object of the invention is to provide such a needle bar driving system.
The invention is based on the idea that instead of providing one or more asymmetric rotating counterweight bodies, a reciprocating linear system can be counterbalanced only with a similarly reciprocating linear system.
In accordance with this idea an improved needle bar driving system has been provided for sewing machines, that comprises an armshaft rotating with a predetermined speed; a needle bar driving assembly driven by the armshaft and having a needle bar performing reciprocating guided linear movement along a substantially vertical axis; a counterweight driving assembly driven by the armshaft; and a counterweight piston driven by the counterweight driving assembly, the counterweight piston performs also a reciprocating linear guided movement along an axis parallel to said axis, wherein the movements of the counterweight driving assembly and of the counterweight piston are always opposite to the movement of the needle bar driving assembly and of the needle bar, wherein the combined torque of the counterweight piston and the counterweight driving assembly acting on the armshaft is substantially equal to the combined torque of the needle bar and the needle bar driving assembly acting also on the armshaft, but these two torques have opposite direction, whereby the armshaft gets substantially balanced.
In a preferred embodiment the counterweight driving assembly comprises a circular disc attached to the armshaft and it has a centre point being offset from the axis of the armshaft, a bearing around the outer surface of the disc having an outer ring, the outer ring is connected to the counterweight piston. The bearing can equally be a sliding bearing and a ball bearing.
In a preferred embodiment the connection between said the ring and the counterweight piston is a link bar connected pivotally to the counterweight piston.
In an alternative embodiment the connection between the ring and the counterweight piston is a link bar; the counterweight piston has a portion of rounded barrel-like profile and the connection between one end of the link bar and the counterweight piston is fixed, whereby during reciprocating movement of the counterweight piston, its axis can get tilted to follow angular displacement of the link bar.
In this embodiment the guided movement of the counterweight piston is provided by a guiding cylinder, and the cylinder and the counterweight piston are made of materials of differing hardness for providing smooth sliding movement of the counterweight piston in the cylinder.
It is also preferred if the counterweight piston has a substantially higher specific mass than the cylinder.
By the invention the objectives can be achieved and a well-balanced system with reduced noise is obtained.
The invention will now be described in connection with preferable embodiments thereof, wherein reference will be made to the accompanying drawings. In the drawing:
a to 1d show respective schematic phase positions of a prior art needle bar driving system;
a to 2d are respective schematic phase position of the needle bar driving system according to the present invention;
Reference is made to
In
This kind of balancing of the movement of the needle bar driving system has the properties described in the introductory portion of the present specification.
Reference is made now to
According to the invention the armshaft 10 moves a second assembly similar to the first one, where instead of the needle bar 16, a counterweight 20, designed as piston is moved, which is guided in a vertical cylinder 21. This second assembly is arranged and driven in a diagonally opposite way to the arrangement and driving of the first assembly. This second assembly comprises a second crankshaft 22 connected and fixed to the armshaft 10, a second link bar 24, and pivot pins 23 and 25 connected to both ends of the second link bar 24. The phase drawings of
The invention cannot be limited to the embodiments shown, the essence of it lies in the perfect balance between the first and second assemblies, both performing alternating reciprocal movements along the same or parallel axes.
In spite of the simplicity of the present invention, its effects are substantial. The reduction of any unbalance of the moving part has reduced the level of noise, there will be no vibration that allows the use of higher working speeds, while the smooth operation does not need heavy and large support, whereby lighter and slimmer sewing machines can be designed.