This exercise is great if you're having grip problems. Get a hex head dumbbell and pick it up by one end.
Obviously, the size of your hands will greatly limit the amount of weight that can be used on this exercise. Once your hand strength increases, you can add weight by using ankle weights attached to the dumbbells. These are generally done for time. Try holding on until you reach failure, or try to beat a certain time records (for example, a ten or twenty second record).
You can do these with both hands or one at a time. Chalk is always used and, as a general rule, no part of your hand or fingers can touch the engraved numbers on the bell. This is cheating!
Get a grip
It does not matter how strong you are if you can't hold on. Grip training is one of the most overlooked aspects of strength and conditioning. Make the difference in your program and don't overlook your need to get a grip.