Counterbalancing means are required to support the weight of the X-ray tubehead as well as to maintain the tubehead in a desired position after it has been moved thereto. Several prior art mechanisms for accomplishing such tubehead counterbalancing employ spring means to counterbalance the tubehead in an upper position and include an arrangement for increasing the resistance of the spring means in lowered positions where additional force is necessary in order to achieve effective counterbalancing.
Other mechanisms employ a plurality of springs. One of the springs however would only be brought into action in lower positions of the tubehead, thus providing substantially uniform counterbalancing of the tubehead in almost any position.
Still other prior art mechanisms employ cam-chain arrangements in addition to spring means. A chain is caused to slide over a cam or roller surface in response to movement of the tubehead to change the tension of the springs in order to compensate for the change in moment of the parallel motion linkage arms associated with the counterbalance mechanism as the tubehead is moved from one position to another.
Each of the abovediscussed prior art mechanisms is somewhat cumbersome, complicated to use, and quite expensive to fabricate. Further, if tubeheads of different weights are to be effectively counterbalanced, then at least two separate adjustments, and even three, must be made by the dentist prior to actual use of the tubehead.