Simultaneous Dead-Centre Linkage
It has discussed in the previous post that for continuous rotation of the Driver-Link, it is essential that the sum of the length shortest and the longest links is equal to the sum of the length of the other two links. There will be a always be a null position i.e. when neither of the links will be a driver or a driven. How do we ensure continuous rotation? The solution to this problem is --- we will have to put some guidance or constraint in place.
In order to understand this, let us take a practical example, a Rail-Rod-Drive.
One of the most important applications of the simultaneous-dead-centre-linkage is the Rail-Rod-Drive, used in Locomotives, to transmit power from one set of wheels to another set of wheels. The skeleton-sketch of the mechanism, showing wheels 2 and 4 with a side-rod 3 are same as four-bar linkage, is posted below for your ready reference;
From the Fig. you can see that when A & B are at positions A' & B' respectively, we have the so called null-position, which means in this position neither of the wheels can be a driver, as a result the mechanism will come a halt. In order to have a continuous we can't afford to be in a situation like this. The solution lies in the smart positioning of the linkages.
You must have noticed that the Locomotive Wheels are fixed to the Axles. In order to overcome the null-position issue, we mount a side-rod on the wheels on the other side of the locomotive with a phase angle..... usually 90° (look at A" & B"), between the driving links or cranks, thereby ensuring, if one side of the rod is on the dead-centre, the other crank acts as the driver. This is how the Rail-side-Rod mechanism functions.
Grashof Linkage, redifined:
In view of the above discussions, let us go back once again to the Four-Linkage-Mechanism and try to understand a few more points. The diagram is reposted below, for ready reference;
Going back to the Grashof Linkage, since the simultaneous-dead-centre -position can't serve the purpose of positive driving. As such, most of the times, whenever we refer to the term Grashof-Linkage, it will be be a four-bar-linkage where the sum of the length of the shortest and the longest will be less than the sum of the length of the other two links.
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