什么限制线性轴承速度？（第2部分）

In第1部分在轴承速度方面的系列系列中，我们研究了限制速度的因素linear recirculating bearings。在此细分市场中，我们将研究限制线性平原轴承速度的因素。

Linear plain bearings operate with sliding contact between the two bearing surfaces, so the limitations encountered with rolling elements aren’t a factor in plain bearing performance. But that doesn’t mean plain bearings have unlimited speed capabilities. Instead, plain bearing speed is primarily dependent on heat generation.

For a linear plain bearing, there is a large contact area between the bearing and the shaft or guide. This allows plain bearings to carry heavy loads, but it also causes them to generate more heat than a bearing with less contact area, such as rolling element bearings. This heat has two effects on the bearing: it can degrade lubrication or the lubricating properties of the bearing, and it can change the clearances between the bearing and its housing or between the bearing and the guide. Both scenarios result in increased friction, greater wear, and reduced life.

Plain bearings have a large contact area between the bearing and the guide.
Image credit: igus

循环轴承在球和导向器之间的接触区域很小。
Image credit: igus

PV因子

Heat is generated by movement, and the amount of heat generated depends upon the load on the bearing, the bearing’s speed, and the coefficients of friction of the materials. This is why linear plain bearings have what is referred to as aPV rating（PV =压力X速度）。PV等级取决于系统从轴承中传递热量的能力。它指定了轴承可以正常运行的负载和速度的最高组合。

因此一个线性滑动轴承operating at a high speed will have a reduced load-carrying capability, and a plain baring carrying a high load will have a reduced speed capability. Note, however, that a plain bearing also has a maximum load capacity rating and a maximum velocity rating. So it’s not possible to carry an infinitely small load and travel at an infinitely high speed, or vice-versa, even if the PV rating would be acceptable.

PV图显示了普通轴承的允许压力和速度之间的关系。
Image credit: igus

尽管PV等级是普通轴承最大速度的关键指标，但轴承材料也起着重要作用。

Material influences

Coefficient of friction has a significant influence on a bearing’s PV rating, and in turn, its maximum speed. The higher the friction coefficients of the materials, the more heat will be generated as the surfaces slide across one another during operation. The choice of materials, of course, determines the coefficient of friction.

Plastic bearings have insulating properties, meaning that heat is retained in the bearing, causing the material to expand and friction to increase as the bearing operates. Aluminum guides, however, have good thermal conductivity and can help transfer heat away from the bearing. Plain bearings made of metal (typically bronze) are often lubricated with oil in high-speed applications. This lubrication reduces friction between the sliding surfaces and aids in cooling. The construction of composite bearings make them good at transferring heat away from the inside of the bearing.

功能图片来源：联邦公路管理