# How much load can a ball screw carry?

There is enormous confusion about this subject. And granted, it is not easy. There are generally two figures given for a particular ball screw, the dynamic load capacity, and the static load capacity. Do they mean that you can put a load up to to the dynamic capacity on the ball screw wile it’s running, or a load up to the static capcity wile it’s not? No to both!

The **dynamic capacity** is just a figure to calculate with. Depending on which definition you are using, it’s either the load for a (theoretical) life of 1 million revolutions (ISO or JIS definition), or for 1 million inches of travel (ANSI definition). But that still doesn’t mean you could run the screw with this kind of load and expect a ballscrew life of one million revs. or inches! Why? The normal maximum operating load of a general use ball screw is about 30% of the dynamic load capacity. Above that, the elastic deformation of balls and races might be too large, which may cause excessive wear and result in premature failure. So, if you run a ball scrtew with a dynamic load equal to it’s dynamic capacity, you may actually not get the life of one million revs. or one million inches that is suggested by the definition. You get less – which is why I call this a **theoretical** life. Confused? Don’t worry, we’ll get there!

A similar problem exists with the definition of the **static capacity**. This figure suggests that you could put a load up to the static capacity on the screw while it’s not running. But that’s not true either! Staying below static capacity onyl ensures that balls and races don’t suffer brinelling (or plastic deformation) of balls and races, but you could easily tear off the flange of the nut, snap the bearing or drive journal of the screw, or even collapse it from exceeding the buckling load. Large balls can carrfy enormous loads without brinelling, so that load may actually be way too much for everything else. But why are they then using such large balls in the first place? Well, they may be necessary to get a high enough dynamic capacity!

OK, by now you must be sufficiently confused to either give up, or to continue reading the post “Dynamic capcity and ball screw life”.