Items related to The constructor; a hand-book of machine design
Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1902 Excerpt: ...to three (see also the following section). The results as determined by calculation are not always practicable for the desired ratchet construction, which shows that the selection of the relation between the parts must be made with judgment, and care taken that those pieces which are subjected to heavy pressure shall not readily be deformed. As the preceding example indicates, the small size of the angle a renders it an important point for consideration. In this case the actual length of d is only about,',T" greater than a + i-f-c-The pressures Q and R act to lengthen d and shorten a, b and c, and if P--100 pounds the difference may readily = jV, so that with only additional wear, a becomes zero, the parts b, c pass the centre, and the ratchet action ceases. This will indicate the conditions linger which the ratchet becomes a useful device. The numerical magnitude of the parts alt c and d can be chosen so as to render the unavoidable wear least hurtful. This may be done with the sliding ratchet, Fig. 713, by making the length c sufficiently great. Il is also important to devise means to prevent the block and pawl from being forced past the centre. A method of accomplishing this i. to substitute for the pin joint between the block and pawl, a curve or cam bearing, as in Fig. 717. It is desirable to examine the value of the coefficient of friction f, in order to increase it at the point 2. This cannot well be done by choice of material, since wood can scarcely be used in many cases, and lubrication of the rubbing surfaces is essential. The application of wedge profiles to wheel and friction block enables greater friction to be obtained, Fig. 718, as in the case of / wedge friction wheels, 196. Instead of the coefficient / we have the value-..--. If J sm ...
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