THKBLK2020-3.6ZZBall Screw Nut Only — 20 mm Shaft, 20 mm Lead, 62 mm OD Flanged, Brush-Sealed, No Preload (Screw Shaft Not Included)

Product Description

What this part is

A THK ball screw nut, model BLK2020-3.6ZZ, from the BLK series of rolled (precision roll-formed) large-lead ball screws. It is the recirculating ball nut half of a ball screw assembly: hardened balls circulate through return channels in the nut and roll in the helical groove of the screw shaft, converting shaft rotation into linear travel of the nut with rolling rather than sliding friction.

Decoding the order code

BLK is THK’s rolled large-lead ball screw series. The first pair of digits, 20, is the screw shaft outside diameter in millimetres. The second pair, 20, is the lead — the nut advances 20 mm per shaft revolution, which is what makes this a large-lead screw: lead equals shaft diameter, giving fast traverse for a given motor speed. 3.6 is the number of loaded ball turns, arranged as 2 circuits x 1.8 turns. ZZ is the seal code — brush wipers fitted at both ends of the nut. Because no preload symbol appears, this is the no-preload build with a nominal 0.1 mm axial clearance.

Scope — what is in the box

The nut only. There is no screw shaft, no end machining, no support bearings, no bearing housings, no nut bracket and no coupling. A ball nut and a ball screw shaft are separate purchases and a nut is only usable on a shaft of matching diameter, lead, thread hand and ball size — this nut needs a 20 mm shaft with a 20 mm right-hand lead in the BLK rolled series. Do not attempt to run it on a ground precision screw of the same nominal size; the groove profile and ball diameter are not the same.

Published specifications

Series: BLK, rolled ball screw, large leadScrew shaft outside diameter: 20 mmLead: 20 mm per revolutionNut outside diameter: 62 mmNut overall length: 45 mmNut form: flanged (double-chamfered flange)Loaded ball circuits: 2 x 1.8 turns (3.6 total)Preload: noneAxial clearance: 0.1 mmSeal: ZZ, brush wipers both endsPositioning accuracy class: C10 range typical of rolled BLK screws; nut running tolerance to THK’s rolled-screw standardApproximate mass: 0.35 kg

What these specs mean in practice

A 1:1 lead-to-diameter ratio is a deliberate choice for speed, not for resolution. At 20 mm of travel per turn, a 1 500 rpm motor gives 30 m/min of traverse; the trade-off is that resolution per encoder count is coarse and the screw is far less able to hold a load against gravity without a brake, because a large lead is much closer to being back-drivable. The no-preload build with 0.1 mm axial clearance is the right specification for one-directional or lightly loaded positioning and for any axis where low drag torque matters more than backlash; it is the wrong specification for a reversing contour-cutting axis, where a preloaded or double-nut screw is used instead. Brush wipers keep swarf and grit out of the ball return without adding the drag of a contact lip seal.

Fitting notes

Ball nuts are not designed to be removed from a shaft without a transfer sleeve. If the nut is supplied loose, keep it on its shipping mandrel until it is on the shaft — running the balls off the ends of the circuit will drop them, and a nut short of balls will feel notchy and fail quickly. Fasten through the flange and make sure the flange face is square to the shaft axis: a cocked nut bracket loads one circuit and shortens life dramatically. Grease with a lithium-soap ball screw grease before first motion and do not rely on the anti-rust oil the part is shipped in.

Interchange guidance

The number is also written BLK2020-3.6ZZNUT (the MISUMI and THK nut-only order code), BLK2020-3.6ZZ, BLK 2020-3.6ZZ and BLK2020. The suffix 5I11 that appeared in earlier copy for this item is a shelf or lot marking, not part of the THK designation, and is kept here only as a search term. Related sizes and builds in the same rolled series are BLK1616-3.6ZZ (16 mm shaft, 16 mm lead) and BLK2020-2.6ZZ (fewer loaded turns, lower rating) — different parts, not substitutes. Ball nuts are not interchangeable across manufacturers: an NSK, Hiwin, Rexroth or Kuroda nut of the same nominal 20 x 20 will not have the same groove radius or ball size and will not run on a THK BLK shaft.