This invention relates to a device for locking rotation of a movable shaft (hereinafter referred to as "a shaft-locking device") used for adjustment of an inclined angle in various kinds of devices such as a display for a word processor or for a personal computer, a headrest of a seat, or a reclining seat, or for various kinds of lids such as for prevention of falling of a toilet lid.
The shaft-locking device is used for adjusting inclined angles in various kinds of devices, FIG. 7 shows a conventional example of a shaft-locking device The shaft-locking device is provided with a movable bracket 31 secured to a device which requires a control of an inclined angle (not shown), a fixed bracket 32 secured to a supporting member (not shown) such as a base stand which supports the device directly or indirectly, a fixed shaft 33 integrally secured to the fixed bracket 32, and a movable shaft 34 which rotates integrally secured to the movable bracket 31. The fixed shaft 33 is a stepped shaft, with a small diameter portion 33a thereof being mounted outwardly and rotatively on the movable shaft 34. The outer diameter of a large diameter portion 33b of the fixed shaft 33 is made to be a same diameter as the outer diameter of the movable shaft 34. A coil spring 35 is inserted around the fixed shaft 33 and the movable shaft 34. The spring 35 is wound so as to be somewhat smaller than the outer diameters of 33 and 34 in a free state and closely contacts the shafts 33 and 34. In this case, both end portions 35a and 35b of the spring 35 forms free edges, the end portion 35a and 35b being inserted outwardly so that each end portion 35a and 35b may be positioned on the movable shaft 34 and on the fixed shaft 33 respectively.
In a shaft-locking device having such a construction, the movable shaft 34 is locked by a friction force of the spring 35 to hold the device at a fixed angle. The adjustment of the angle is performed by causing a slip between the spring 35 and the movable shaft 34 by adding an outer force more than the friction force, thereby rotating the movable shaft 34. Since the rotation in the direction of the arrow TL of the movable shaft 34 (rotation in the winding direction of the spring 35) is a rotative direction which shrinks the coil diameter of the spring 35, the friction force increases to form a locking torque, while the rotation in the direction of arrow Ts (rotation in the winding return direction of the spring 35) is a rotation in a direction which enlarges the diameter of the spring 35, and the friction force decreases thereby causing a slip torque. Since the locking torque is large compared with the slip torque, the device is maintained at a fixed angle by the locking torque.
In the conventional shaft-locking device, although it is possible to perform a locking against rotation in the winding direction, in the rotation in the opposite direction (winding return direction) against the winding direction, the spring 35 enlarges its diameter and the friction force decreases to lessen the locking force. Accordingly, in order to securely lock both rotations in the winding and its opposite direction, another shaft-locking device wherein the winding direction of the spring 35 is opposite is necessary. This causes a complicated construction
Further, in the conventional shaft-locking device, the winding of the spring 35 on a movable shaft 34 is not always uniform. This is because the spring 35 is combined with the movable shaft 34 so as to have a fixed winding interference. Such tendency is most accentuated at an end portion 35a of the spring 35 on the movable shaft 34. The end portion 35a of the spring 35, as shown in FIG. 8, contacts with the peripheral surface of the movable shaft 34 at point P, but at the coil side spaced from the contact point P the spring 35 does not contact with the movable shaft 34 as a reaction of this contact In such an outwardly extending state of the spring, the locking torque of the spring 35 cannot be constantly restrained, thereby causing an unstable locking problem.