Spiders are important mechanisms in large reciprocating engines that guide the piston rod up and down, reducing wear and maintaining engine rotation stability. They consist of a pair of crosshead guides and are not used in smaller engines. Spiders also make servicing large diesel engines easier.
A spider is a mechanism found in larger reciprocating engines that guides a piston rod up and down. This helps to eliminate side pressure on the piston, which would otherwise be present in larger engines without a spider. Spiders are an extremely important part of large reciprocating engines, as they not only reduce wear created by lateral pressure, but also help regulate and maintain engine rotation stability. This stability is directly affected by the stability of the reciprocating motion of the piston.
Like most components found in larger reciprocating steam or diesel engines, the spider itself is a relatively simple device that is entirely mechanical and consists of very few parts. While the design can vary between engines, they all work the same way; The device itself consists solely of a pair of crosshead guides, where the crosshead is free to move up and down. The crosshead is connected to the piston rod and crosshead bearing of the engine, which are guided up and down by lubricated guides.
Although crossheads play a very important role in large engines, smaller engines, especially internal combustion engines found in automobiles, do not use them. One of the reasons for this is because the design itself does not require a crosshead; The pistons of gasoline internal combustion engines do not have a rod, since the piston and the crankshaft are directly connected by the connecting rod. Another reason is because side pressure is much more tolerable on a small engine than a large engine, since everything is on a much smaller scale. A larger engine without crossheads would wear very easily, as the side pressure placed on the pistons at higher RPMs would create a large amount of wear.
In addition to eliminating side pressure, spiders have other benefits. One of these is found in internal combustion engines that use crossheads, such as diesel engines. Pistons in reciprocating engines need occasional maintenance or replacement, so they need to be completely removed from the engine. On rodless reciprocating engines, doing this would involve removing the connecting rod from the crankshaft, which can be a difficult process. With a piston rod present, however, removing the piston is as simple as disconnecting the piston rod from the connecting rod. This makes servicing large diesel engines much easier.
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