A METHOD AND AN ARRANGEMENT FOR CONTROLLING THE TENSION OF A BAND SAW-BLADE

Abstract: An arrangement for controlling the tension in the saw-blading (5, 6) of a bandsaw equipped with an electrically controllable blading-tensioning arrangement by means of which the blading can be stretched so as to adjust the tension therein to a desired level. To this end, the arrangement includes automatically operating devices (8-13) which are operative to determine whether or not the blading-tension is optimal during a sawing operation. These devices are intended to sense and determine mutually different parameters, such as specific blading-tension, disturbances in the movement pattern of the blading, the linearity of the saw cut, etc. The arrangement also includes a microprocessor (9) which is operative to process the electric signals corresponding to the sensed parameters. The microprocessor is programmed to send control signals to the blading-tensioning arrangement when the microprocessor establishes deviations from limit values programmed therein with respect to the parameters or fluctuations of these parameters, thereby to adjust blading-tension in a manner to reduce these deviations or fluctuations. The invention also relates to a method for controlling the tension of sawblading.

Description: A METHOD AND AN ARRANGEMENT FOR CONTROLLING THE TENSION OF A BAND SAW BLADE The present invention relates to a method of controlling the tension of the sawblading of a bandsaw with the aid of an electrically controllable blade-tensioning device by means of which the sawblading can be stretched so as to adjust the blading to a desired tension. The inven- tion also relates to a device for carrying out the method. When sawing timber into planks and boards with the aid of bandsaws, the sawblading is normally set to saw over- dimensioned pieces of timber in order to compensate for inaccuracies occurring in the actual sawing operation. Sawing can be achieved with a high degree of precision, provided that the teeth of the sawblading are sharp, deviations in accuracy will occur as the sharpness of the teeth decreases. The sawblading is normally tensioned so as to be flexible, wherewith endeavours are made, inter alia, to reduce the aforesaid deviations, by lower¬ ing the infeed speed and therewith reduce the forces transmitted through the teeth on the sawblading during a sawing operation. A prime object of the present invention is to increase sawing accuracy while retaining a desired high infeed speed, and to maintain or prolong the useful life of a sawblading, by optimizing blade-tension with each sawing occasion. This is achieved by increasing blade-tension instead of lowering the infeed speed when there is a tendency towards deviations in a sawing operation, since such an increase is found to improve the sawing accuracy of the blade. When this higher blade-tension is no longer required, the blade is relaxed towards its original, starting or basic tension, thereby preventing the blade from being subjected to unnecessary loads liable to reduce the useful lifespan of the blade. One prerequisite in this respect is that the saw is equipped with a sawblading tensioning arrangement whose power output can be controlled in a rational manner with the aid of electronic pulses. Present day micro processors can be used to control proportionally both the gas pressure and hydraulic pressure of known blade- tensioning devices, for instance devices of the kind described and illustrated in Swedish Patent Specifica- tion 7312757-3 and in WO 88/03850. Thus, in order to achieve the aforesaid objects with the aid of a blade-tensioning device of the kind taught, for instance, by one of the aforesaid patent specifica- tions, it is necessary to select on each occasion an optimum blade-tensioning level which will provide an accurate sawcut without the occurrence of problems in the form of fatique cracks in the sawblading. Stresses in the saw blade or blading are mainly in the form of the tensile stress created by the tension force in the cross-section of the saw blade and by the bending tension occurring as the blading bends around the saw pulleys or wheels. In addition to these stresses, stresses are also created in the blading in conjunction with sharpening the teeth by grinding. During sawing action, the sawblading is also subjected to external forces, such as to the effect of the cutting force on the tips of the sawteeth, to the effects of thermal loads and vibrations, and to the effects of torsional and lateral loads caused by the blading itself or because the workpiece moves laterally. The following measures are examples of those measures which can be taken for the purpose of achieving optimum blading-tension values and of improving sawing accuracy. The specific blading-tension can be calculated and used for control purposes, on the basis of a sensed cross- sectional area of the blading which is changed inter alia each time the sawblading is ground, and on the basis of the tensioning force in the blading. Contact- less inductive sensors can be used to detect momentary disturbances in or deviations from the intended movement path of the blading caused, for instance, by heating of the blading, vibrations generated therein or lateral loads thereon, the signals from said sensors being registered and utilized to initiate increased tensioning of the sawblading. Variations in the pressure exerted by the sawblading on the blading guides can also be sensed and recorded, for the same reason. Elevated stresses in the tips of the teeth of sawblading, caused by blunting of said tips, result in greater in- accuracies in the saw cut. This can be monitored with the aid of known image pro¬ cessing apparatus constructed to produce signals corre¬ sponding to deviations from an imaginary saw cut. Among other things, this will enable the saw to be operated at relatively low blading-tension provided that the sawblading is sharp, among other things in order to avoid unnecessary loading of the sawblading, this ten¬ sion being gradually increased in a manner to maintain sawing accuracy as the sharpness of the sawteeth de¬ creases. Sensing and calculating devices intended, inter alia, for sensing and calculating the aforesaid parameters can be employed to achieve substantially optimum blading-tension with each sawing operation, wherein 5 attention is paid, inter alia, to the specific tension in the gawblading, the sharpness of the sawteeth, the hardness of the wood being sawn, etc. By permitting the blading-tension to return towards a 10 starting value upon completion of each sawing operation, for instance subsequent to sawing each log, there is afforded the advantage that the tension in the blading will not remain at a high value, e.g. a value determined by a log containing a very large number of knots, when 15 such high tension levels are not required for sawing a following log, e.g. a log which comprises a far less dense type of wood. Consequently, in accordance with the present invention, 20 a method of the kind described in the opening paragraph is mainly characterized by the steps of determining during a sawing operation, with the aid of automatically operating means, whether the blading-tension is optimum or not; determining said tension by sensing and deter- 25 mining mutually different parameters, such as specific blading-tension, disturbances in the pattern of blading movement, saw-cut linearity, etc.; processing electrical signals corresponding to said parameters in a micro¬ processor, when deviations from parameter limit values 30 programmed in the microprocessor or fluctuations in the parameters are established, transmitting control signals from said microprocessor to the blading-tensioning device such as to adjust the blading-tension; determining by means of said automatically operating means when the 35 blading-tension is optimum; and terminating the control signals to the blading-tensioning device when optimum blading-tension is established. The method will therewith enable the tension in the sawblading to be maintained at an optimum level for the sawing process concerned and to be adjusted conti¬ nuously in response to changed conditions. The microprocessor will preferably be programmed to cause the blading-tension to return towards a starting value prior to commencing each new sawing operation. This will guarantee, inter alia, that the requisite blading-tension is re-set when commencing to saw each log, thereby eliminating the risk of the saw being operated with an unnecessarily high blading-tension. The starting value towards which -che blading-tension is returned at the end of each sawing operation can be increased progressively with the time over which saw¬ blading is used, thereby avoiding the need to make unnecessarily high adjustments to blading-tension at infrequent intervals. The main characteristic features of apparatus construc¬ ted to operate in accordance with the inventive method are set forth in the following apparatus claims. The invention will now be described in more detail with reference to exemplifying embodiments illustrated in the accompanying drawings. Figure 1 illustrates schematically an installation which includes two mutually cooperating bandsaws provided with means for controlling blading-tension in accordance with the invention. Figure 2 is a schematic, horizontal view of the instal- lation illustrated in Figure 1. Figure 3 is a schematic side view of one bandsaw illu¬ strated in Figure 1. The reference numerals 1 , 2 and 3, 4 in Figure 1 iden¬ tify respectively the upper and the lower saw pulleys or wheels of two bandsaws. A respective sawblading 5 and 6 runs over each pair of pulleys and is stretched until the desired tension is achieved in the sawblading. Stretching of the sawblading is effected by subjecting the upper saw .pulleys 1 , 2 to a force F with the aid of a hydraulic piston-cylinder device or like device mounted on the saw frame. The tensioned sawblading will preferably be slightly resilient or springy, so as to reduce loading shocks. Since the bandsaws and the manner of their construction may have any prior known form which will enable the blading-tention to be adjusted, the bandsaws and their construction will not be described in detail here. Examples of bandsaws of this kind are found described and illustrated in the aforesaid specifications, namely the Swedish Patent Specification 7312757-3 and WO 88/03850. The reference numeral 7 identifies a log being sawn, whereas the reference numeral 8 identifies a device for sensing and adjusting the tensioning force.F, this device being connected to a microprocessor 9. The micro¬ processor also receives signals from a sensing device 10 ^ operative to sense the sawblading area and a device 11 which is operative to detect any disturbance in or deviation from the ideal movement path of the saw- blading, for instance disturbances in the form of vibrations, twists and the like. As illustrated schema- tically in Figure 2, the devices 11 may consist of two inductive sensors each located on a respective edge of the sawblading but out of mechanical contact therewith. The pressure exerted by a tensioned saw- blading against blading guides 12 can also be detected or sensed, inter alia, for the purpose of indicating fluctuations in said pressure. The reference 13 iden¬ tifies a sensing arrangement of the video-camera type which is used to determine the linearity of the saw cut 14 and deviations of the saw cuts from ideal saw cuts. Although the sensing devices have only been shown in respect of one bandsaw, it will be understood that normally both saws will be provided with mutually the same devices. When using equipment according to the aforegoing, the microprocessor 9 receives data relating to, inter alia, possible fluctuations in the abutment pressure of the blading against the blading guides and possible twist¬ ing of or vibrations in the blading, and also data relating to the accuracy of the saw cut, or calculates such data on the basis of sensed measuring values, inter alia on the values of specific blading-tension. The microprocessor compares these parameters with limit values stored in the microprocessor, and when necessary sends a signal to the blading-tension adjusting device 8, indicating that the tension in the sawblading should be increased, in order to eliminate a particular distur- bance or to improve sawing accuracy. Thus, in accordance with the aforegoing, the tension in the sawblading is adjusted, inter alia, in accordance with the logs concerned and the prevailing sharpness of the sawteeth. When sawing of a log is completed and the sawblading idles for a short period of time, the tension in the sawblading is returned towards a preset starting value. When sawing of the next log commences, the ten¬ sion in the sawblading is again adjusted by application of the te sioning force F in dependence on the condi¬ tions which prevail in respect of this particular log. Thus, the tension in the sawblading is adjusted indi¬ vidually with each log and within limits which consider¬ ably optimize the accuracy of the saw cut and the useful length of life of the sawblading. The microprocessor 9 is preferably programmed to vary the basic level to which the sawblading is tensioned, so that the degree of tensioning is increased slightly with time during the use of the sawblading. This increase in blading-tension shall be based on the knowledge, gained by experience, of the extent to which blading- tension must be increased in order to maintain the desired sawing accuracy as the sawteeth lose their sharpness. The base value or starting value can also be preset with respect, e.g., to the dimensions of the logs or wood being sawn. The microprocessor can also be used to store data relat¬ ing to the loading and operating time of the sawblading, the total load to which the sawblading is subjected being compared with an experience-value stored in the microprocessor and the microprocessor being programmed to indicate when sawblading should be replaced even when the sharpness of the sawteeth is satisfactory. It will be understood that the invention is not limited to the aforedescribed and illustrated bandsaw arrange¬ ment and that modifications can be made within the scope of fhe following claims. For instance, other measuring values can be determined and used for controlling the tension of the sawblading. Furthermore, one or more of the aforementioned measuring values can be omitted.

Claims: Claims 1. A method for controlling the tension in the saw¬ blading of a handsaw with the aid of an electrically adjustable blading-tensioning device by means of which the blading can be stretched so as to adjust the blading- tension, c h a r a c t e r i z e d by the steps of determining during a sawing operation, with the aid of automatically operating means, whether the blading- tension is optimum or not; determining said tension by sensing and determining mutually different parameters, such as specific blading-tension, disturbances in the pattern of blading movement, saw cut linearity, etc.; processing electrical signals corresponding to said parameters in a microprocessor, when deviations from parameter limit values programmed in the microprocessor or fluctuations in the parameters are established, transmitting control signals from said microprocessor to the blading-tensioning device such as to adjust the blading-tension; determing by means of said automati¬ cally operating means when the blading-tension is optimum; and terminating the control signals to the blading- tensioning device when optimum blading-tension is established. 2. A method according to Claim 1, c h a r a c t e r - i z e d by adjusting the tension of the sawblading automatically in dependence on the cross-sectional area of the sawblading in use at that time. 3. A method according to Claim 1 or 2, c h a r a c - t e r i z e d by returning the blading-tension to a basic starting value prior to commencing each new sawing operation, with the aid of said microprocessor. 4. A method according to Claim 3, c h a r a c ¬ t e r i z e d by increasing the basic value of said blading-tension in correspondence with the time over which said sawblading is used. 5. A device for controlling the tension in the saw¬ blading (5, 6) of a handsaw with the aid of an electri¬ cally controllable blading-tensioning arrangement by means of which the blading can be stretched to adjust • blading-tension to a desired level, c h a r a c t e ¬ r i z e d in that the device includes sensor means (10-13) operative to sense and determine different parameters during a sawing operation, e.g. such para¬ meters as specific blading-tension, disturbances in the ' ideal -Bbvement pattern of the blading, the linearity of the saw cut, etc. ; in that the device also includes a microprocessor (9) which is supplied with electric signals orresponding to said parameters and which when establishing deviations from limit values relating to said parameters programmed in the microprocessor or fluctuations in the parameters is operative to send control signals to the blading-tensioning arrangement such as to produce a smooth or continuous change in said blading-tension until said parameters again fall within said limit values.

Inventors: JANSON KURT, LOENNAAS BERTIL, JANSON, KURT, LOENNAAS, BERTIL
Applicants: ERIKSSON AB A K, AB A K ERIKSSON
Application year: 1990
Main classification: B23D55
IPC codes: B23D55, B23D59