Typically wheels are attached with self-locking nuts at the trucks on roller skates. But there is another way.
Nuts keep the wheels safely at their place. Everybody, who experienced a lost wheel while skating, knows about the relevance of a tight connection of bearings and wheels on the axle. But wheels can also be held on the axles without threads and nuts, using a quick release axle system.
Instead of a nut, quick release axle systems (commonly known as clip or flip axles) are using a splint, attached at the tip of the axle. Until the 1970s, skaters used wheels made from wood or phenolic resin, which wear out quickly and were susceptible for breaking. In long-distance competitions, skaters had to change their wheels during the race. The quick release system was invented by Italian Enzo Boiani in 1965, founder and owner of the manufacturer BOEN (BOiani ENzo), famous for its equipment for speed skating at that time. He made switching wheels very fast. So that wheels could be changed during the race.
The splint is secured by a spring-loaded ball inside the axle and prevents the splint from coming loose.
Sound like the perfect locking system for skate wheels. However there are some disadvantages. The splint has only two positions: Open and close. Adjustment of the tolerance of the wheel on the axle is limited, compared to the fine-adjustment, which can be done by turning the self-locking nut on a threaded axle. With clip axles the improvement of the tolerance have to be made by washers of different thickness between the truck and bearing. Nevertheless a small space for turning the splint is needed, so a slight gap with a ticktack of the wheels is unavoidable.
With the gaining popularity of Roller Derby, the wish for a faster switching of the roller skates wheels had arised again. So some manufacturers have rediscovered the clip axles, like Chaya, and Mota, or still offer this smart idea for locking a wheel on an axle, as BOEN, or Sure-Grip.