Case History: hollow shaft

Hollow shaft in EN-GJS500-7 spheroidal graphite iron for car wheel balancers

Methods and techniques for producing a hollow shaft in spheroidal graphite iron

In our fifth case history, we describe the project of the hollow shaft in EN-GJS500-7 spheroidal graphite iron for car wheel balancers – which are commonly found in mechanics workshops – (Nexion brand) of Nexion S.p.A

In which industries is the hollow shaft mostly used?

Customers who require a hollow shaft in spheroidal graphite iron for their wheel balancers are part of the industrial tool sector for tyre specialists and car mechanic workshops which work closely with the on- and off-road vehicle (e.g. to change tyres of trucks) and agriculture vehicle sector (e.g. to change tractor tyres).

Benefits (in terms of quality and quantity) of spheroidal graphite iron compared to steel

-8.3% weight of the final component

By redesigning the component, it was possible to decrease the weight of the final component by almost 10%.

Minimum impact on machinability

The net-to-shape design of the raw EN-GJS-500-7 spheroidal graphite iron part minimised machining thanks to the reduction of stock allowance and the use of cores.

Less waste material (-82.3% weight of the raw part)

A new shape of the raw component in spheroidal graphite iron that closely resembles the machined component made it possible to significantly cut down on waste material (-82.3% of the weight of the raw part between graphite iron and steel) given that the previous version was machined starting from a solid C40 rolled bar.

Reduction in costs

Less machining combined with less waste material also had a positive impact on the level of reduction in the production costs of the final component.

Increase in performance

The solution in EN-GJS-500-7 proved more than satisfactory in replacing the C40 steel (from rolled bar).

Development of the application for the Corghi brand wheel balancers of the Nexon group

The hollow shaft for wheel balancers is used specifically to produce Corghi wheel balancers. These machines are required to balance tyres, an essential operation required to ensure that the vehicle is correctly aligned to be driven on the road. This type of hollow shaft is used to transmit motion from the electric transmission to the wheel being balanced.

The problem to solve and the solution in GJS “EN-GJS-500-7” spheroidal graphite iron

The customer’s goal was to improve the performance of the component, which was initially made from a grade C40 steel rolled bar. This technological solution called for multiple machining to remove excess material, make the through hole and thus, obtain the finished version. The process led to the significant waste of material, plus high machining costs.

In light of these production inefficiencies, Nexion S.p.A. decided to optimise the production process by obtaining a raw component that resembled as closely as possible the final shape of the product after machining (net-to-shape), with adequate tensile strength to withstand the loads involved. Before opting for the solution in spheroidal graphite iron, the customer had also considered the possibility of producing the shaft in two steel parts.

Thanks, however, to the natural casting capacity of spheroidal graphite iron to obtain a net-to-shape cast part and the possibility of using a core to make the internal through hole, the solution of producing the hollow shaft in EN-GJS-500-7 spheroidal graphite iron proved to be the best option. It was thus possible to reduce the amount of machining required to obtain the final shape, resulting in a significant economic benefit for the customer.

The benefits obtained by producing a hollow shaft in spheroidal graphite iron

Starting material

Material

Rp0,2

Fatigue limit
σAG; PS50%

C40 - Rolled bar

260 MPa

530 MPa

156

The new solution assessed to achieve the goal

Material

Rp0,2

Fatigue limit
σAG; PS50%

EN-GJS-500-7

320 MPa

500 MPa

170÷230

330

Tensile and fatigue test specimens obtained from Ø25 mm separately cast samples. The fatigue tested in rotating bending (R-1) on Ø6,5 mm machined samples according to Short staircase method e Run out at 5ML cycles. In reality, the workpiece has a variable thickness.

The customer’s goals were the following:

To cut production costs of the steel component (economic saving)
To reduce material waste (material removed during machining)
To retain the same mechanical performance

To achieve this, it was essential to find a technological solution that guaranteed the minimum performance necessary for the loads involved to which the hollow shaft could be subjected. The solution in EN-GJS-500-7 spheroidal graphite iron, thanks to its mechanical properties, proved more than satisfactory to undertake this change in material and production technology.

Manufacturing methods and techniques with a comparison between steel and graphite iron

Given the need to convert the previous solution obtained from a C40 rolled steel bar to the new version in cast EN-GJS-500-7 spheroidal graphite iron, this hollow shaft project involved a major co-design collaboration between the technical departments of Zanardi Fonderie and Nexion S.p.A.

The intrinsic properties of spheroidal graphite iron made it possible to approach the raw component with a net-to-shape logic right from the design phase. Zanardi Fonderie’s technical department, in collaboration with the customer’s technical department, designed a raw component with a shape that closely resembled that of the machined component. It is possible to design with this logic thanks to the excellent “castability” of spheroidal graphite iron, i.e. the ease with which the liquid metal produces complex shapes. This made it possible to minimise the stock allowance and to “take” the material only where it is needed, thus eliminating all the areas where the material is not required, for example by using cores. The use of a core, in fact, allowed us to create the through hole and thus limit machining operations only to where strictly necessary.

When you use a solid steel bar, this technology requires you to start with a (solid) steel cylinder and then, remove a large quantity of material to obtain the final shape of the component which, in our case, also included the through hole by removing chips. All this leads to significant machining costs and longer production times while wasting a lot of material that cannot be reused.