On the face of it, keeping fluids contained seems like a simple job. Your fridge alone probably has a dozen or more trivial examples of liquids being successfully kept where they belong, whether it’s the plastic lid on last night’s leftovers or the top on the jug of milk. But deeper down in the bowels of the fridge, like inside the compressor or where the water line for the icemaker is attached, are more complex and interesting mechanisms for keeping fluids contained. That’s the job of seals, the next topic in our series on mechanisms.
Packing it In
One of the simplest seals is packing, or compressing some sort of flexible material into a space to control the flow of a fluid. Packing probably dates to at least the time when humans began making boats more complicated than a simple dugout canoe, in response to the fact that it’s really difficult to keep water from leaking between two pieces of wood. Ship seams have been caulked with fibers like hemp and cotton soaked in pitch or tar for millennia.
A more complex seal, in the form of a stuffing box around the tiller post of an inboard rudder, extended the concept of caulking into a more dynamic environment. A tiller post is a vertical shaft that penetrates the bottom hull of a vessel and connects the submerged rudder to a tiller or other steering gear. To staunch the flow of water through such a gaping wound, a stuffing box was built inside the hull around the shaft and packed full of old rope fibers, rags, bits of sailcloth — anything fibrous and compressible. Soaked with tallow or fat for lubrication and water repellency, the stuffing was compressed with a gland around the shaft that was tightened down under screw pressure or using wedges. Stuffing boxes are still used to this day for all kinds of shafts that penetrate a vessel’s hull, including where the propeller shaft exits the stern, in which case the assembly is called a stern gland.
Maritime tradition has stuck with compression packing. While far from perfect, compression packing seals are cheap and easy to engineer, and so still see service to this day. Compression seals are used in water pumps and as a seal around the pump shaft to keep the process fluid controlled. While the materials used have come a long way, with highly engineered Teflon and graphite packing replacing hemp and lard, the basic technique is still the same — squeeze the packing as tightly as possible around the shaft to make it hard for fluid to penetrate. Hard, but not impossible — compression packing always leaks to some degree. Packed joints therefore require constant maintenance, including frequent tightening of the gland to maintain compression and eventually replacement of the old packing with fresh material.
Sealing the Deal
Ongoing maintenance costs and the tendency of compression packing to wear out the shaft they are wrapped so tightly around are two of the biggest drawbacks to these seals, and so other types of seals for rotating shafts were invented. Your car engine has a ton of seals, but the ones that can cause you to have a really bad day when a mechanic tells you you’ve “blown a seal” are the crankshaft seals. There are usually two, one where the crankshaft exits the crankcase at the rear to connect to the transmission, and one at the front where the crank pulley that runs the water pump, alternator, and other engine accessories exits. Seals are needed around the crankshaft because the crankcase is full of oil; the seals keep the oil inside the engine while allowing the crankshaft to rotate and run the car.
Crankshaft oil seals are known as radial shaft seals or lip seals, where a flexible material is formed into a ring that fills a space between the stationary part (the crankcase in our example) and a rotating shaft (the crankshaft journal). Like compression packing materials, lip seals were once made from natural substances like leather, but today elastomeric substances like Viton and Buna N are used. Most modern oil seals are composite mechanisms, too, with the elastomer bonded to a metal case that can be pressed into a recess in a crankshaft in an interference fit.
Radial shaft seals seem simple enough, but there are several critical factors in designing a seal that’ll work. Aside from choices like material compatibility with the process fluids and heat ratings, the geometry of the flexible lip is crucial to the design. Looked at in cross-section, the lip forms a triangular profile with the apex touching the shaft at a single contact point. The angle of the lip on the wet side of the seal must be steeper than the angle on the dry side, so that hydraulic pressure from the process fluid (oil in our crankshaft example) tends to force the seal tighter around the shaft. Process fluid pressure is a major consideration as well — too much and the lip can deform enough to increase the contact area, causing premature wear and failure. That’s why we’re warned that too much oil in an engine can be just as bad as too little — more volume means more pressure, leading to the classic “blown seal” symptom of leaking from either the front or rear of the engine.
For viscous process fluids like grease, a plain elastomeric lip seal might suffice. But thinner fluids generally need extra circumferential force to help keep the lip in firm contact with the shaft. Such seals are likely to incorporate a garter spring into the lip to preload the seal on the shaft and to help make up for any irregularities in the shaft. Garter springs also help compensate for any radial oscillations of a shaft.
We’ve only touched on a few of the hundreds of types of seals for both dynamic and static applications here. The engineering behind them is pretty interesting, though, especially when you realize that it all started with a box of greasy rags.
Featured image source: American High Performance Seals