A CNC axis that started getting tighter and noisier after six months. The ball screw was a C5, 32 mm diameter, 10 mm lead, with a double-nut preloaded carriage. The customer greased it every month with a lithium grease they had on the shelf. The screw’s drag torque doubled. The axis lost positioning accuracy. The customer thought the preload had collapsed. The preload was fine. The grease was the problem.
A precision ball screw runs on a thin oil film (or a light grease film) between the balls and the raceway. The grease must stay fluid at operating temperature and must not thicken. A general-purpose lithium grease thickens as it ages, and its thickener (soap) separates. The thickener packs into the raceway. The balls push through thick, sticky grease instead of rolling. The drag torque rises. The screw gets tighter, then it skips under acceleration.
What the right grease looks like
Ball screw manufacturers specify a grease with a specific NLGI grade (usually 0 or 00 — semi-fluid) and a specific base oil viscosity. A semi-fluid grease (NLGI 00) flows like warm honey. It coats the balls and stays in the raceway. A standard NLGI 2 grease (the tub stuff) is too stiff for a recirculating ball nut. The balls can’t recirculate through it. The customer’s lithium NLGI 2 grease was the wrong consistency from the start.
The fix: drain the old grease (flush with a light oil), then regrease with the manufacturer-specified product — in this case an NLGI 00 lithium grease with ISO VG 68 oil. The drag torque dropped back to spec. The axis ran quiet again. The customer now greases with the right product every three months instead of every month.
The grease amount matters more than the interval
Too much grease is as bad as the wrong grease. A ball nut packed full of grease builds internal pressure — the balls push against the grease, the nut heats up, and the grease thins and leaks. The rule: apply grease until it just appears at the seal, then stop. Most people pump in 3x too much. The excess grease heats the screw and makes it run tighter.
The other mistake: greasing a vertical screw. On a vertical axis, gravity drains the grease out of the nut. Vertical screws need a thicker grease (NLGI 1-2) or a much shorter regrease interval. The grease manufacturer’s recommendation for horizontal screws doesn’t apply.
Oil vs grease for precision screws
Grease wins on retention and cleanliness — it stays in the nut and doesn’t fling off. Oil wins on low drag torque and high speed. The general split: under 20 m/min, grease is fine and preferred. Over 20 m/min, or for high-precision feedback axes, use oil (ISO VG 32-68) delivered in small pulses. A grease-lubricated screw at 40 m/min will heat up and lose accuracy. The customer’s axis ran at 15 m/min — grease was fine, just the wrong grease.
The maintenance check
Every regrease, check the drag torque: disconnect the motor, push the table by hand, feel the resistance. It should be smooth and uniform. If it’s notchy or tight, flush and regrease. Also watch the nut temperature after an hour of running — a hot nut (over 50°C) means the grease is wrong or the preload is too high. The drag torque and temperature checks catch 90% of ball screw lubrication problems before they become accuracy problems.
A ball screw doesn’t need “any grease” — it needs the right NLGI grade and viscosity. The wrong grease packs into the raceway and tightens the nut. Use the manufacturer’s spec, apply until it just appears at the seal, and check the drag torque at every regrease. The screw wasn’t worn out; it was full of the wrong grease.
