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Why does a radial engine need a master rod? Why can't slave rods be the only rods? Would this spider bearing (around 1:05 in the video) work for an IC engine, why or why not?

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  • $\begingroup$ The video you link to does not have a master rod and nothing found in my research says that a radial engine will not function without the use of a master rod. There may be some practical reasons for a master like, like assembly, lubrication, possibly balancing, or something along those lines. $\endgroup$ – GisMofx Feb 26 '16 at 22:41
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Having a master rod means that the bottom bearings of the connecting rods follow a fixed path throughout the cycle. If they are all attached to the crankshaft via a spider bearing it the spider bearing itself has an extra degree of freedom to rotate around the crank bearing, having a master rod constraints this rotation.

Because the con rod bottom bearings are centred at some radius from the centre of crank bearing they will exert a torque when the line of action of the cylinder is not on a direct line through the centre of both. The master rod resists this torque which is ultimately transmitted to the cylinder preventing the con rod bottom bearings from rotating around the centre of the crank bearing relative to the centreline of the master connecting rod.

Depending on the geometry this could potentially lead to collisions between the con rods and cylinder walls and is likely to waste power by making the cylinders fight against each other.

A master rod would not be required if the geometry of the engine is such that the torques on the con rod bottom bearings balance out naturally, this will depend on overall geometry, crank throw, stroke, number of cylinders etc.

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The geometry of the radial engine prevents all the rods from being directly connected to the crankshaft. All of the cylinders are in a single plane, thus the connecting rods are also in that same plane.

https://en.wikipedia.org/wiki/Radial_engine

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  • $\begingroup$ This isn't clear as my why the master rod is used. Also, the example video in the question does not utilize a master rod. Also, wikipedia's animation shows a radial engine without a master rod. Please clarify. $\endgroup$ – GisMofx Feb 26 '16 at 22:30

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