# Permissible Rotational Speed

#### Critical Speed of the Screw Shaft

When the rotational speed reaches a high magnitude, the Ball Screw may resonate and eventually become unable to operate due to the screw shaft’s natural frequency. Therefore, it is necessary to select a model so that it is used below the resonance point (critical speed).

Fig.13 on Permissible Rotational Speed Diagram shows the relationship between the screw shaft diameter and the critical speed.

If determining the critical speed by calculation, it can be obtained from the equation (7) below. Note that in this equation, a safety factor of 0.8 is multiplied to the result.

#### 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 N _{2} |
Guideline for maximum rotational speed |
|||
---|---|---|---|---|---|

Precision | Caged Ball | Model SBK (SBK3636, SBK4040, and SBK5050) |
Large lead | 5000 | |

Model SBK (Other than the above model numbers and the small size model SBK *) |
4230 | ||||

Models SBN-V (Medium), HBN-V | Standard lead | 5000 | |||

Models SBN-V (Small), HBN, and SBKH | 5000 | ||||

Model HBN-K and HBN-KA | 1810 | ||||

Models SDAN-V and SDA-V | Standard lead/Large lead | 5000 | |||

Full-ComplementBall | Models SDA-VZ and SDAN-VX (shaft diameters ø 28 to 63) |
Standard lead/Large lead | 4480 | ||

Models SDA-VZ (shaft diameters ø 10 to 25) |
5000 | ||||

Model WHF | Super lead | 5000 | |||

Model WGF | 5000 | ||||

Models BLW, BLK, BLR, BNS, BNS-B, BNS-A, and NS | Large lead | 5000 | |||

Models BIF-V (Medium), BNFN-V(Medium), and BNF-V (Medium) | Standard lead | 4950 | |||

Models BIF-V (Small), BNFN-V(Small), and BNF-V (Small) | 5000 | ||||

Models BIF, DIK, BNFN, DKN, BNF,BNT, DK, MDK, MBF, BNK, and DIR | 5000 | ||||

Rolled | Full-ComplementBall | Models WTF and CNF | Super lead | 4440 | |

Models BLK and BLR | Large lead | 4440 | |||

Model BTK-V | Standard lead | 5000 | |||

Models JPF, BNT, and MTF | 5000 |

N_{2} : Permissible rotational speed determined by the DN value (min^{-1})

D : Ball center-to-center diameter

(indicated in the specification 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 (N_{1}) or the permissible rotational speed determined by the DN value (N_{2}).

If the service rotational speed exceeds the guidelines for maximum rotational speed, contact THK.

## Point of Selection

- Conditions of the Ball Screw
- Conditions of the Ball Screw
- Estimating the shaft length
- Selecting lead･Selecting a shaft diameter
- Method for Mounting the Ball Screw Shaft
- Permissible Axial Load
- Permissible Rotational Speed
- Selecting a Nut
- Calculating the permissible axial load
- Studying the Service Life
- Studying the Rigidity
- Studying the Positioning Accuracy
- Studying the Rotational Torque
- Studying the Driving Motor