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Duro-Chrome 699 Dual-Pawl Ratchet
During the 1930s and 40s much effort was devoted to improving ratchet mechanisms, especially for the "round-head" designs with fine-tooth actions. While others looked for ways to make finer teeth, the engineers at Duro Metal Products came up with a strikingly novel idea. Instead of adding more teeth to the gear, they added a second pawl to the mechanism, with the teeth of one pawl offset from the other by half a pitch. Then as the ratchet turned to get a new "bite", the pawls would alternately drop into position, thereby effectively doubling the number of teeth. The inventor W.H. Odlum and collaborators were awarded patent #2,395,681 in 1946.
The photograph below shows an early example of this design, the Duro-Chrome model 699 1/2-drive ratchet. The ratchet is marked "U.S.A." and "Pat. Pend.", and the overall length is 10.5 inches. The "Pat. Pend." notation indicates that this is an early production model; based on the patent filing and issue dates, the likely manufacturing date is in the range 1944-1946.
The dual-pawl action can be heard clearly when the ratchet is turned slowly, with the two pawls making slightly different clicking sounds as they fall into place. The Duro catalogs advertised this as having "80 tooth action" without describing its 40-tooth gear and dual pawls.
One interesting aspect of the construction is that the ratchet has been permanently assembled by compressing flanges in the head. This technique was invented by Duro in the early 1930s (as patent #1,902,878) and used for a series of ratchet models. (The patent also appears to have been used by Hinsdale; see for example the H-12-R Ratchet.)
Although the dual-pawl mechanism was a great idea, this ratchet model was apparently not very successful as a product. Two factors probably contributed to its limited success: first, the patent design required two different pawls, one with the teeth offset, and having to make two precision parts would have increased the manufacturing cost.
Secondly, the permanent assembly negated one of the potential advantages of the round-head ratchet design, the simple replacement of the ratchet mechanism as a unit. This could have been significant when the manufacturer offered a lifetime warranty for its tools, as it would mean replacing the entire unit instead of offering (selling!) an inexpensive renewal kit. As a result of these issues (or others), by 1955 Duro-Chrome had already discontinued this model. (The 1955 catalog still illustrates this ratchet model, but the price list substitutes a more conventional Duro-Chrome 675 Ratchet with a 52-tooth design.)
Although not a big success for Duro-Chrome, the dual-pawl patent did pave the way for great products from other companies. In the mid 1950s, W.J. Johnson at J.H. Williams & Co made a brilliantly simple (or simply brilliant) improvement to the design. His ratchet used symmetrical dual pawls, but with an odd number of teeth, thereby gaining the same benefit with a simpler mechanism. This resulted in the Williams S-52 model, arguably one of the best ratchets of all time. And some years later, H.J. Haznar at Moore Drop Forging patented an improved shift mechanism and added a quick-release button, features that were used for the excellent Craftsman V-series dual-pawl ratchets.
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