Sprag clutch elements consist of a cage, sprag number in relation to the shaft size and a spring to preload the friction contact between the sprags and the mating parts. Sprag clutch elements are not self centering and require additional bearing support for proper function.
We feel obliged to point out that there is a highly important influence of design and structure of the mating parts for defining the torque capacity of any Free-wheel clutch.
There is no standard torque capacity for a clutch valid at any design. The most important parameters for defining the torque capacity of a clutch are the material to be used for customer’s parts (shaft and housing) and their wall thickness.
The values of torque capacities shown in the data sheets reflect the following situation:
- solid shaft
- material of shaft and housing: ball bearing steel 100 Cr 6 eq. AISI E 52100; code 1.3505; hardened and tempered; HRC = 60+4; martensite structure; max. alternating bending strength σBW = 750 N/mm2
- wall thickness of housing D/DL = 1.4 (Series 8000)
D/DL = 1,5 (Series 400)
All torque moment specifications “M” in the data sheets correspond with the transmittable nominal torque of each Free-Wheel Clutch. The torque moment includes a service factor of 1.2. Plastic deformation will not take place until the torque moment reaches 1.2 times M, caused by momentary overload conditions. With different specifications, different wall thickness, peak load conditions and higher temperature the nominal torque will be decreased. The diagrams below show the influence of the material on the nominal torque depending on the wall thickness. Here are some examples:
σBW = 400 N/mm2 34 CrMo 4V; 16 MnCr 5 – case hardened HRC = 60+4; Eht = 1.3
σBW = 600 N/mm2 20 MnCr 5 – case hardened HRC = 60+4 ; Eht = 1.3
σBW = 750 N/mm2 100 Cr 6 – hardened and tempered; HRC = 60+4
Further information (i.e. service factors) could be found in our catalog or simply contact our technical staff in Nuernberg without any obligation.
Please note that nonferrous material at the mating parts reduces torque capacities reasonably. If you considered to use nonferrous material please contact our technical staff first.
This information covers also our complete units and ball bearing sizes. Hardening and grinding of the mating parts is not necessary in this case.
The sprags may lift off of the inner race due to centrifugal forces at a certain maximum speed. In this case the clutch might not work properly. The maximum idling speed is shown in the data sheets to each clutch.
Please note that the sprags could lift off only if they are disengaged. If the sprags are in engaged position they could not lift off even at very high speeds.
When torque is transmitted the sprags are in a fixed position between inner- and outer race. Under these conditions – theoretically – there is no speed limit. In backstopping and overrunning applications the influence of centrifugal forces has to be considered.
Tests showed that with oil lubrication series 8000 sprags surf on a hydrodynamic lubricating film under constant conditions (v = const.) with rotating shaft. The clutch itself turns very slowly under these conditions and so centrifugal forces do not become effective. The same situation could be expected with grease lubrication at a little bit higher speed level.
The rotating speed of the clutch itself depends on size, viscosity and temperature. With a hydrodynamic lubricating film only minimal wear could be expected.
Please note that in indexing applications due to alternating conditions a hydrodynamic lubricating film cannot be created. Please contact our technical staff for some advise in these kind of applications.
Lubrication: Insert elements are rust protected only. Further information could be found under indications of lubrication.