Each style has 3 type tread material DIK:Rubber,grey,cone ball bearing. DYK:Rubber,black,electric conductive,cone ball bearing. PJP:Thermoplastic rubber ,precision ball bearing, housing made of zinc-chromatised pressed steel, double ball bearing swivel head, wheel axle with nut, wheel center made of zinc-chromatised steel or PP with threadguardsFor hospital bed or other applications.