A yarn fault detector suitable for mounting on a main frame of a knitting machine or on a side creel. The detector has two yarn guides on a guide frame that is biased to an operative yarn guiding position but movable overcenter to a yarn releasing position. End portions of a freely pivotable stop bar contact the yarn guides in the operative position of the guide frame to assure adequate contact with both guides. A variable bias is applied to the guide frame by means of a loop of flexible material on a rotary shaft, and a spring under tension between the loop and an eccentric cranked portion of the guide frame, the tension being dependent on the length of the loop wrapped around the shaft by rotation of the shaft. The loop and shaft are made by a one-piece moulding operation.The invention relates to yarn fault detectors, sometimes known as "top detectors," for detecting breaks and excessive tensions in yarns supplied to textile machinery such as knitting machines, from creels mounted at the top of such machines. Such detectors include lightweight pivotally mounted detector wires which rest on a yarn passing over a pair of guides and which, when the yarn breaks, fall to complete an electrical circuit for actuating a stop motion relay to stop the machine. Excessive tensions in the yarn wound on the creel are detected by pivotally mounting the guides and urging them to an operative yarn guiding position by means of an overcentre device. The top detector in use is positioned so that any tension in the yarn passing from the creel and over the guides to the machine tends to urge the guides to move overcentre to a yarn releasing position, in which case the detector wire falls to complete the circuit for the stopmotion relay as above. The guides are generally provided with ceramic guiding surfaces on a wire frame, and the bias urging the guides overcentre to the operative yarn guiding position is preferably variable.One problem that has been experienced when using such detectors is that when there is rapid decrease in the rate of demand for yarn by the machine, the yarn passing from the creel to the top detector tends to overshoot the guides and the resulting slackening of the yarn can cause the yarn to become wrapped around the guide nearer the creel rather than passing smoothly over it. It has been proposed to avoid this occurrence by causing the guide nearer the creel to contact a lug on the frame of the top detector, so that there is no space between the guide and lug to permit the yarn to depart from its intended path. The provision of one such lug is perfectly satisfactory when the top detector is designed solely for mounting on the frame of the machine. It is however desirable for the top detectors to be mountable optionally on side creels or on the frame of the machine, using the same snap-on fitment at one side of the top detector for both mountings. When the top detectors are designed in this way it is found that the direction of yarn travel is reversed when the detector is turned round and mounted on a side creel, relative to that when it is mounted on the frame of the machine. Thus the lug referred to above is no longer associated with the guide nearer the creel, and it is necessary to provide an additional lug for cooperation with the other of the guides. The disadvantage of such a construction is that it is very rare for the guides both firmly to contact their associated lugs at all times. What in fact happens is that the frame mounting the guides is not quite in alignment with the plane of the lugs, so that only one of the guides is in contact with its associated lug. This tendency is much more pronounced when the tension biasing the guides upwardly to their operative positions is very light. If only the guide remote from the creel is firmly in contact with its associated lug, the yarn can become wrapped around the above guide in precisely the manner indicated above.Another problem associated with such fault detectors is the difficulty of producing a variable bias on the guide frames. In the past the coil springs in such fault detectors have been tensioned by engaging one end of the spring in a U-loop of filament the free ends of which are entrapped in the central split of a cotter pin. A knob is mounted on an end of the cotter pin by means of a grub screw, and turning of the knob causes the filament to wrap around the cotter pin, thus shortening the length of filament between the cotter pin and spring and extending the spring so as to increase its tension. There are problems in assembling and using such a device. Assembly is difficult as the free ends of the U-loop of filament have to be inserted in the split of the cotter pin. Generally this is made easier by first heat-welding the free ends to one another, but this in itself is a time-consuming operation. When assembled, the device has to be set to provide an appropriate minimum or maximum tension to the spring so as to provide meaning to calibrations on the knob. This is achieved by gripping the bent-over end of the cotter pin with pliers and turning the pin until the desired reference (maximum or minimum) tension is obtained, and then tightening the grub screw to fix the knob on the opposite end of the cotter pin with the knob in the desired angular condition. This too is a time-consuming operation.OBJECTS OF THE INVENTIONIt is an object of the present invention to provide a top detector which avoids the above problems of entanglement of the yarn in the yarn guides.It is a further object of the present invention to provide a top detector of simple and economical construction in which the bias on the guide frame can be easily adjusted.It is a further object of the present invention to provide a top detector in which the calibration of the bias on the guide frame can be re-zeroed in a simple and rapid manner.Other objects will become apparent from the following description.SUMMARY OF THE INVENTIONThe invention provides a yarn fault detector for textile machinery, comprisinga housing,detector wire means pivotally mounted on said housing,a pivotable guide frame pivotally mounted on said housing and movable overcentre between an operative yarn guiding position and a yarn releasing position,means within said housing biasing said guide frame to its yarn guiding or releasing positions,a pair of yarn guides on said guide frame,a fixed guard frame having side guards, one at each side of said pair of yarn guides, extending from said guide frame for further guiding yarn through the detector and,a freely pivotable stop bar on said fixed guard frame for contacting both of said yarn guides when said guide frame is in its operative position.Because the yarn guides contact a freely pivotable stop bar rather than fixed lugs, there is always good contact between the guides and the stop bar sufficient to prevent the yarn from passing therebetween, irrespective of the direction of passage of the yarn. When the pivotal axis of the stop bar is midway between the yarn guides, the force opposing the bias urging the guide frame to its operative position is shared equally between the two guides.The guides are preferably ceramic guides on a wire pivotable guide frame. The stop bar may be mounted by any desired means, such as by a pivot pin or by having its ends freely slidable in slots in a frame of the top detector and its mid portion engageable with a fulcrum when the guide frame biases the stop bar thereagainst. The stop bar may be of any desired cross-section.The invention also provides a yarn fault detector for textile machinery, comprisinga housing,detector wire means pivotally mounted on said housing,a pivotable guide frame pivotally mounted on said housing and movable overcentre between an operative yarn guiding position and a yarn releasing position,a one-piece moulding of a resilient plastics material having an integral loop portion and a shaft portion and being rotatably mounted within said housing,a control knob externally of said housing, associated with said one-piece moulding for effecting rotation thereof,a cranked portion of said guide frame internally of said housing, andresilient means between said cranked guide frame portion and said loop portion for biasing said guide frame towards its yarn guiding and releasing conditions with a bias that is dependent on the amount of said loop portion that is wrapped around said shaft portion. In use, the guide frame is biased to its operative position with a bias that may be varied by rotating the rotary shaft to wrap more or less of the flexible loop thereon. The loop portion is advantageously pre-stretched to reduce its tendency to creep. The rotary shaft may be rotatable by means of a knob splined to the shaft and removable axially, so that the knob can be removed axially, turned to any desired angular orientation and pushed back onto or into the shaft. This facilitates rapid setting of the knob to a given reference position of maximum or minimum tension.Preferably the rotary shaft is made from a resilient plastics material such as nylon and also comprises an integral serrated disc the serrations of which are adapted in use to ride over a projection, such as a ridge or dimple, to give a resistance to turning.