As the name suggests, it is a flexible joint between two rotating parts that maintains
the same input and output rotational speed despite the changing angle of joint.
What Is Inside
Essentially it is very much like a ball joint but with ball bearings sitting in grooves
to stop the inside and the outside of the joint from spinning freely of each other.
Apart from the driveshaft ball and the socket you will find a set of bearings, a floating
collar that keeps the bearings in place, a retaining clip (in some cases) and a lot of grease.
CV Joint Grease Info
It's obvious why the joint needs grease, they can spin at thousands of revolutions a
minute and encounter a lot of internal pressure (which is mostly converted to heat).
The grease used is molybdenum disulfide. It is sold under countless other brand names.
You often read about people warning on the dangers of CV joint grease and how you
should be very careful when you handle it. There is no reason to panic, here is the truth.
Molybdenum disulfide (MoS2) is actually very safe compared to the grease used decades ago.
Obviously you aren't going to want to get it on your skin but that's simply because it's messy,
it will not be absorbed and it is rare for skin to react badly to it. (Don't test it out though).
The estimated lethal dosage through consumption is nearly 500g.
Inhalation can be a problem if you are working with it in a room barely big enough to get in.
The only two big hazards are easy to keep away from. Don't burn it or get it in your eyes.
CV Rubber Boot
The rubber boot is there for two main reasons, to stop dirt getting into the grease
and contaminating the joint, and to stop the grease from flying out during use.
If the rubber has perished or split, you will get the grease coming out and dirt and water
getting in. It is important the joint stays uncontaminated due to the stresses involved.
If a split boot is left you will get increased wear of the joint to the point where it either seizes
up during operation or destroys itself. This could lock the wheels or damage a brake pipe.
It is worth noting that a split CV boot is an MOT fail so worth keeping an eye on.
Outside Joint Disassembly