In this post we discuss and compare the torsional displacement applied into two different materials driveshafts. We evaluate a Stainless Steel (AISI 304) solid shaft and a hollow Carbonfiber shaft. Both shafts have universal joints installed in both ends and one end clamped while the rotational force is applied to the other.
This test focuses on SS driveshafts extensively used in food machinery.
Despite the automotive industry, in food machines torque is usually delivered to one side, while the other tends to rotate freely, carrying its load respectively. This occurs because of free space lack since this type of machines should be compact into their chassis without having moving parts over them and over the goods processed. It is therefore essential to fit all components into small compartments usually beneath. These shafts tend to bend over time and the obvious solution to manufactures is to replace them with even larger Diameter ones.
Therefore, this type of evaluation is made to compare performance and also choose that CF shaft that could effectively replace the SS into these machines. Of course, ideally the rotating force should be delivered equally to both endings or to the shaft midpoint – considering the shaft and loads symmetrical to the Midplane.
Filament Winding Process
Usually wet-Filament Winding is applied to a extrudable rotating mandrel, which is pre-covered with a release wax or agent (better option than release film). Resin systems are Epoxy based, with hardeners. Nowadays, there are various food grade epoxy matrices to be used into the food industry and comply with industry's specifications. The winded product is then wrapped with a shrink tape and it is cured at 80oC for about 4hrs and postcured at 140oC for another 4hrs approx, increased by a pre-determined ramp Temp. Autoclave curing oven is the best option, but most times not available. However, the shrink tape will still do the job.
|Material||SS (AISI 304)||CF-Toray T700S (PAN)|
|Wall thick (mm)||-||3|
|Mass (gr) – without joints||13930||571,45|
|Density||8E-06 Kg/mm3||1,43E-06 Kg/mm3|
|Ultimate tensile strength||505MPa||577MPa|
|Thermal conductivity||0,0162 W/(mmC)||0,0105 W/(mmC)|
|Thermal expansion coefficient||1,73E-05 /C||9,93E-06 /C|
|Specific heat||500 J/(KgC)||1130 J/(KgC)|
*Carbonfiber sriveshaft is considered to be Filament Wound with a helical angle of +/-30, +/-45, +/-55 and +/-75degrees respectively and two hoop layers. Fiber volume content approx. 30%.
Safety factor achieved 5.527 for SS and 5.348 for CF, both of which are beyond desired minimum value.
The carbonfiber hollow shaft achieved similar performacnce with the solid Stainless steel, despite the large mass difference. We should also mention that the CF accelerates faster than the SS due to its lighter weight and its capable to reach a greater Torque-To-Weight ratio.
The hollow CF shaft will eventually failure by means of catastrophically buckling after its withstanding bend while the SS shaft will remain bended. In some cases this is a better option since upon damage the machine will stop operating. On the other hand a bended shaft may keep rotating affecting other components too.
While, an interesting demo video of the above study can be found here. Published on 30 Jan 2013 by mechanicalTV27 on YouTube. Courtesy of BBC Engineering Connections - Formula One.
Text and images by george vakfaris.