When selecting a ball screw, most people only look at diameter and lead. Is that enough? Most of the time, yes. But a couple of times, I got burned because I only looked at those two.
Later, I went back and summarized the pitfalls. There are two parameters that are listed in the catalog, but many people (including my past self) never look at them. Hopefully this helps you avoid my mistakes.
The First Parameter: Critical Speed
Let me start with a real story. A couple of years ago, I was helping a customer select a screw for an X‑axis with 1200mm stroke and a required speed of 60m/min. I calculated the load, lead, and motor torque – everything seemed fine. But when we got the machine running and hit 50m/min, the whole table started vibrating like a tractor.
It took two days to figure out that I had completely ignored the screw's critical speed. That screw was 25mm in diameter, fixed‑supported mounting. Its critical speed was around 1900rpm. To reach 60m/min, it needed 2400rpm – well over the limit. We swapped to a 32mm diameter screw, the critical speed went up, and the problem disappeared.
What is critical speed? It's the speed at which the screw resonates. Think of spinning a pencil – too fast and it jumps. The longer and thinner the screw, the lower its critical speed. Always check whether your required rpm exceeds the critical speed. The formula is in every catalog – don't skip it.
The Second Parameter: Allowable Axial Static Load
Another trap. A customer was building a vertical axis (Z‑axis) with about 300kg of load. They chose a 32mm screw. By the rated dynamic load, it was more than enough. But six months later, the screw was bent.
I took it apart – it wasn't worn, it was actually bent. Why? On a vertical axis, when the machine is off, the load keeps pressing down on the screw. Over time, the screw can bend. This is called the allowable axial static load. It's in the catalog, but again, many people ignore it.
Lesson learned: For vertical axes, don't just look at the dynamic load – check the static load too. Go bigger on diameter, especially when the stroke is long and the load is heavy.
Two Questions to Ask Yourself Every Time
Now, whenever I select a ball screw, I ask myself two extra questions:
- Does my maximum rpm exceed the critical speed? Especially for long‑stroke machines.
- For a vertical axis, will the screw creep (bend) under static load when the machine is off?
You won't find answers to these in a catalog – you have to calculate and think ahead.
An Installation Detail: Lock Nut Loosening
The lock nut at the end of the screw secures the bearings and eliminates axial clearance. Over time, machine vibration can loosen it. When the lock nut loosens, backlash increases suddenly – many people think the screw is worn, but it's just a loose nut.
What to do: Tighten the nut with a hook spanner, then apply a punch mark or thread‑locking compound. For high‑speed machines, check it every three months.
A Few Honest Words
Selecting a ball screw isn't just picking a diameter and lead from a catalog. Critical speed, allowable static load, and lock nut tightening – overlook these, and the machine will tell you later. I've made these mistakes, so you don't have to.
Zhejiang Baili Guide Rail offers ball screw selection calculation and installation technical support. If you're unsure what size or length fits your conditions, send us your parameters – we'll help you calculate.