THKBLK2020-3.6ZZBall Nut Only for 20 mm Shaft x 20 mm Lead Rolled Large-Lead Ball Screw, 39 mm OD x 45 mm, Flanged, Sealed Both Ends, No Preload

Product Description

What this part is — nut only

This is the ball nut only from a THK BLK-series large-lead rolled ball screw. There is no screw shaft, no support units, no end bearings and no motor coupling in this listing. That is the single biggest source of buyer disappointment on ball screw listings anywhere, so it is worth stating up front rather than in the fine print.

You can confirm it from the part number itself. A complete THK ball screw assembly carries a shaft-length code, so the full designation would read something like BLK2020-3.6ZZ+1500L C7 T, where the +1500L gives the shaft length and the trailing codes give accuracy grade and shaft finish. There is no +____L code here, which means the nut alone. MISUMI catalogues exactly this item as BLK2020-3.6ZZNUT.

Correcting the source data

Our previous description for this record referenced “model 5I11.” That was a stray code carried over from a different record and has no bearing on this part. It has been dropped. The correct and complete designation for this item is BLK2020-3.6ZZ, nut only.

Decoding the order code

BLK — THK rolled (thread-rolled, not precision-ground) large-lead / Transport Ball Screw series. Single nut, no preload, DN value 70,000.
20 (first) — 20 mm screw shaft diameter.
20 (second) — 20 mm lead.
3.6 — number of loaded ball circuits, made up as 2 rows × 1.8 turns.
ZZ — brush-type wiper seals fitted at both ends of the nut.

What rolled and no preload mean in practice

This is the crux of the part, and it decides whether the screw suits your machine.

A rolled ball screw is made by thread-rolling the shaft rather than grinding it. That is a much cheaper process and it puts the screw in the C7 or C10 accuracy class, rather than the C0 to C5 classes you get from precision grinding. BLK is a transport and general-motion screw — it is designed to move things from A to B efficiently, not to position a machine tool axis to a few microns.

Just as important: this is a single-nut, no-preload design, and THK publishes an axial clearance of 0.1 mm for it. That clearance is backlash. A single nut with no preload always has some, by construction. If your application needs bidirectional repeatability tighter than 0.1 mm without a linear encoder closing the loop around the screw, this is the wrong screw and no amount of careful fitting will fix it — you want a preloaded or double-nut precision-ground screw from the BNK family instead.

If, on the other hand, you are moving a gantry, a shuttle, a lift, a transfer slide or a tool changer, the 20 mm of travel per revolution and the low cost per metre are precisely why you would choose this series, and the 0.1 mm clearance is irrelevant to the job.

The 1:1 lead-to-diameter ratio and what it buys:
A 20 mm lead on a 20 mm shaft is a 1:1 lead-to-diameter ratio, which is a genuinely large lead — most general-purpose ball screws run 5 mm or 10 mm on a 20 mm shaft.

What it buys you is speed. Twenty millimetres of linear travel per motor revolution means high traverse rates from a modest motor speed, and the published permissible rotational speed of 3,370 min⁻¹ puts a lot of travel per minute on the table. It also reduces the reduction ratio, so you need less motor RPM for a given feed.

What it costs you is thrust and holding. A large lead converts less torque into axial force, so for the same motor torque you get less push than a fine-lead screw of the same diameter. It also makes the screw markedly more back-drivable — an axial load on the nut will happily spin the screw. On a vertical axis that matters a great deal: plan on a motor brake or a mechanical lock, because this screw will not hold a suspended load on its own.

Published ratings and mounting data

Nut outer diameter D: 39 mm (g6)
Flange diameter D₁: 62 mm
Overall nut length L₁: 45 mm
Flange thickness H: 10 mm
B₁: 27.5 mm
Mounting: 4 bolt holes of 5.5 mm on a 50 mm PCD, Tw 46 mm
Lubrication hole: M6 (N₁ 5 mm)
Basic dynamic load rating Ca: 7.7 kN
Basic static load rating C₀a: 22.3 kN
Axial rigidity K: 210 N/µm
Axial clearance: 0.1 mm
Permissible rotational speed: 3,370 min⁻¹
Nut mass: 0.35 kg
Seals: ZZ brush wipers, both ends
DN value: 70,000

Mating shaft data, for reference when you source the shaft separately: ball centre diameter d⃗ 20.75 mm, thread minor diameter 17.5 mm, shaft mass 2.18 kg/m.

Quantity and configuration

One nut. Nut only. To build a working axis you will also need a BLK2020 screw shaft cut and end-machined to your length, a fixed-side and supported-side bearing unit, and a coupling. Those are all separate items.

Installation and setup notes

A ball nut removed from its shaft can lose its ball complement. If you are fitting this nut to a new shaft, it must go on using the correct assembly sleeve or arbor — slide it on freehand and the balls drop out of the circuits, and reloading a nut by hand is a miserable job that rarely ends with all circuits correctly filled.
Match the nut to a BLK2020 shaft specifically. A shaft of the same 20 mm nominal diameter from a different series, or with a different lead, will not mate — the ball centre diameter, groove profile and lead all have to agree. Same nominal size is not the same screw.
Bolt the flange to a face square to the screw axis and concentric with it. Misalignment between the nut flange and the shaft axis loads one side of the ball circuits and shortens life sharply.
Use the M6 lubrication hole. Ball screws are grease- or oil-fed and a dry screw fails fast at 3,370 min⁻¹.
The ZZ brush wipers keep swarf out of the circuits. Do not run the nut with the wipers removed in a dirty environment.

Interchange guidance

Written as BLK2020-3.6ZZ, BLK2020-3.6ZZNUT (the MISUMI nut-only number), BLK-2020-3.6ZZ and BLK 2020 3.6 ZZ; the base number is BLK2020. Related THK series that are not the same part:

BTK series — transport screws with a smaller lead-to-diameter ratio. More thrust, less speed, different nut.
BNK series — precision ground ball screws in the C0–C5 accuracy classes, available preloaded. This is what to buy if you need positioning accuracy rather than transport speed.
The same nut without seals — BLK2020-3.6 with no ZZ suffix. Same mechanics, no brush wipers, and it will not give you the same contamination protection.