Permissible Rotational Speed
Critical Speed of the Screw Shaft
At high rotational speeds, the ball screw may resonate due to the screw shaft’s natural frequency and eventually become unable to operate. Therefore, it is necessary to select a model that can be used below the resonance point (critical speed).
Fig. 13: Permissible Rotational Speed Diagram shows the relationship between the screw shaft diameter and the critical speed.
The critical speed can be calculated by equation (7) below. Note that in this equation, the result is multiplied by a safety factor of 0.8.
DN Value
The permissible rotational speed of the ball screw must be obtained from the critical speed of the screw shaft and the DN value.
The permissible rotational speed determined by the DN value is obtained using the equations (8) to (17) below.
| Model No. | Permissible rotational speed determined by the DN value N2 |
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|---|---|---|---|---|
| Precision | Caged Ball | Model SBK (SBK3636, SBK4040, and SBK5050) |
Large lead |
 |
| Model SBK (Other than the above model numbers and the small size model SBK *) |
 |
|||
| Models SBN-V (Medium), HBN-V | Standard lead |
 |
||
| Models SBN-V (Small), HBN, and SBKH |
 |
|||
| Model HBN-K and HBN-KA |
 |
|||
| Models SDAN-V and SDA-V | Standard lead to super lead |
 |
||
| Full-Ball | Models SDA-VZ and SDAN-VX | Standard lead to super lead |
 |
|
| Model WHF | Super lead |
 |
||
| Model WGF |
 |
|||
| Models BLW, BLK, BLR, BNS, BNS-B, BNS-A, and NS | Large lead |
 |
||
| Models BIF-V (Medium), BNFN-V(Medium), and BNF-V (Medium) | Standard lead |
 |
||
| Models BIF-V (Small), BNFN-V(Small), and BNF-V (Small) |
 |
|||
| Models BIF, DIK, BNFN, DKN, BNF,BNT, DK, MDK, MBF, BNK, and DIR |
 |
|||
| Full-Ball (ISO standard compliant) |
Models EPB-V, EBB-V (2806 to 8020) | Standard lead |
 |
|
| Models EPB-V, EBB-V (1605 to 2512) | ||||
| Rolled | Full-Ball | Models WTF and CNF | Super lead |
 |
| Models BLK and BLR | Large lead |
 |
||
| Model BTK-V | Standard lead |
 |
||
| Models JPF, BNT, and MTF |
 |
|||
N2: Permissible rotational speed determined by the DN value (min-1)
D: Ball center-to-center diameter
(indicated in the dimensional tables of the respective model number)
When considering the rotational speed, the permissible rotational speed is regarded as the lower of the following maximum rotational speed guidelines: the critical speed of the screw shaft (N1) or the permissible rotational speed determined by the DN value (N2).
If the service rotational speed exceeds the guidelines for maximum rotational speed, contact THK.
Selection Criteria
- Operating Conditions of the Ball Screw
- Operating Conditions of the Ball Screw
- Selecting a Screw Shaft
- Selecting lead･Selecting a shaft diameter
- Mounting Procedure for the Ball Screw Shaft
- Permissible Axial Load
- Permissible Rotational Speed
- Selecting a Nut
- Calculating the Permissible Axial Load
- Considering the Service Life
- Considering the Rigidity
- Considering the Positioning Accuracy
- Considering the Rotational Torque
- Considering the Driving Motor
