Design and selection of flexible shafts
Prerequisite to the selection of flexible shafts is a knowledge of the operational requirements, i. e. in particular the maximum torque and the power to be transmitted, as well as the operating speed.
Selection sequence:

  1. Clarify operational requirements.
  2. Select flexible shafts using the tables and power transmission chart.
  3. Select protective hose and end connections.


Components of a flexible shaft





The torque to be transmitted and the matching shaft core size (and derived therefrom also the protective hose) can be determined on the basis of the power and the speed using the formula given below. The values indicated in the size tables apply for speeds of 20% of maximum speed and with straight installation conditions. At higher speeds the maximum torque decreases proportionally.
The maximum admissible torque as specified in the table may not be exceeded, because it can cause permanent deformation of the shaft.





Power rating

Each shaft has a maximum transmittable power rating which is shown in the chart below.





Direction of rotation
Flexible shafts differ both in their construction and in their direction of winding. A left-wound shaft (related to its outermost layer) can transmit a higher torque in clockwise direction than in counterclockwise direction; a right-wound shaft can transmit a higher torque in counterclockwise direction. Depending on the shaft construction, the shaft can have approximately the same strength in both directions.

1. Outermost layer left-wound for operation in clockwise (right-hand) direction

2. Outermost layer right-wound for operation in counter-clockwise (left-hand) direction



In-service geometry
Since the in-service geometry affects the transmittable power, flexible shafts should be installed with as large radii as possible.

Influence of bend radius on transmittable power
The torques and speeds indicated in the table are applicable for operation in a relatively unbent condition. If the shaft is operated strongly bent, the values will fall below those indicated. The diagrams given below show guideline values for the maximum admissible power transmission in function of operating radius.



Minimum bend radius
The maximum admissible operational shaft bend radius is designated the “Minimum bend radius”.






In-service radius
Since flexible shafts lessen the need for exact alignment of motor and tool unit, simplification of the tool design is achieved. In the case of parallel misalignment of drive and tool unit the bend radius can be calculated as follows:





Special operating conditions
Attention must be paid to the environment in which the flexible shaft will be used. For example, extremely high or low temperatures, moisture, corrosive influences, dust, magnetic fields, vibrations, etc. Such factors can influence the choice of materials for core and casing and their fabrication. Our technical departments will gladly offer their advice.


Important criteria

The following clarifications of choice of core, casing end coupling should also be made:

  • Longevity
  • Flexibility
  • Continuous or intermittent operation
  • Fast toolholder change
  • Shaft related to coupling or slide length
  • Difference in length of core and casing



Torsional deflection
Corresponds to the angle of torsional deflection of a shaft under load. The desired maximum degree of torsional deflection is one of the parameters governing the required diameter and type of the shaft. The angle of torsional deflection is proportional to the torque and the length of the shaft.


Torsional deflection

Y = Torsional deflection at applied torque [°]
S = Stiffness factor according to table [°/10 Ncm/m]
Mb = Applied torque [Ncm]
L = Length of shaft [m]


Breaking torque
Under this load, the shaft will twist or break. The figures are given in the sizing tables.


Reduction in degree of torsional deflection
The degree of torsional deflection in a flexible shaft varies proportionally with the torque. To keep the deflection, and the load as well, as low as possible, the flexible shaft should be operated at highest possible speed. If the speed is increased, the gearbox should be on the motor (drive) side; if it is reduced, it should be on the tool side.

Speed reduction



Speed increase





The maximum speed of a flexible shaft is indicated in the table. The admissible speed is a function of the installation situation and of the torque to be transmitted.


Shaft guiding
As a rule of thumb: The shaft should be guided from 20 to 30 x . Not every protective casing is suitable for all applications (e.g. friction heat). For the shaft diameter to casing inside diameter ratio, as a rule of thumb 1:1.2 can be assumed.


In practice, depending on application and shaft diameter, flexible shafts with lengths of up to 15 m have proven successful.


In the case of high-speed rotary cores and cores longer than 5 to 8 cm, a protective casing is recommended to assure:


  • Safety protection of personnel and equipment.
  • Ability to handle shaft while running.
  • Continuous guiding and support of the core. (Torsion shafts can be secured with guide rings.)
  • Smooth operation.
  • No “looping” of the rotating core during operation under torsional load.
  • Shock absorption and absorption of push-pull forces.

The service required depends on the prevailing working conditions. With normal use without adverse conditions (moisture, heat, dust, etc.), the core should be cleaned and greased after about 200 operating hours. Under extreme conditions, e.g. wetness, cleaning and greasing are recommended after 50 operating hours.


X     Last number:
1 for operation in clockwise (right-hand) direction
2 for operation in counter-clockwise (left-hand) direction

  1. Min. bend radius: The bend radius cannot be smaller than this value.
  2. Torsional deflection: Torsional deflection angle over 1 m shaft with torque of 10 Ncm.
  3. Breaking torque: At this load the shaft will break.
  4. Max. torque: Values applicable for straight shaft at speeds of 20% of maximum rated speed.