Embedded Files
Requirements
Requirements
1. Client:
The Ball screw needs actuate the crossmember assembly throughout the testing process. During this process the screw needs to maintain its length and alignment.
2. User:
The ball screw must be easy to inspect and perform maintenance on.
3. Safety:
The ball screw’s capacity must meet or exceed the maximum capacity of the tensile testing machine’s maximum force. The ball screw should be encased to prevent injury during operation
4. Standard:
The ball screw must meet or exceed the testing parameters outlined in ASTM D638-14.
5. Law:
The ball screws design and manufacture should not infringe upon any existing intellectual property or patents.
6. Environment / Sustainability:
All manufacturing processes in the production of the device shall conform to all applicable standards.
7. Cost Efficiency:
It must be cost effective, while still meeting all requirements and standards
8. Weight:
The weight of the ball screw is not a critical requirement.
Specifications
Specifications
Force Capacity
0.5 [kN]
Vertical Test Space
726 [mm]
Testing Speed Range min/max (Return)
0.05-2500 [mm/min]
(1875)
Frame Axial Stiffness
8.5 [kN/mm]
Maximum Force at Full Speed
0.5 [kN]
Maximum Speed at Full Force
2500 [mm/min]
Maximum Power Requirement
250 [VA]
Material and Process Selection
Material and Process Selection
From the above requirements and specifications for the ball screw, we are able to translate these into a set of constraints and objectives. The critical constraints for the ball screw are its ability to withstand a 500 N axial load, the ability to withstand an induced moment, performing at the specified strain rate and accomplishing this while complying with the ATSM 638-14 standard. Of particular importance for this component is section 5.1.6 of ATSM 638-14, it states that the entire assemblies elastic longitudinal strain does not exceed 1% of the total longitudinal strain between the two gage marks on the test specimen at any time during the test and at any load up to the rated capacity of the machine. With these constraints in-mind, due to budget restrictions, our objective function is to minimize cost.
In addition to the aforementioned stiffness constraint, the material needs to have high hardness to ensure the ball bearings do not cause local plastic deformation on the thread during loading. A high hardness will also help to reduce the wear rate constant of the ball screw, increasing its service life. The screw then needs to have a high fracture toughness, due to the dynamic and cyclic functionality. This ensures that while the material has a high modulus and strength, the propagation of cracks within the material will be minimized. Finally, the screw should be corrosion resistant to ensure smooth and unimpeded operation and ensuring the longevity of the component.
Similar requirements apply to a testing machine on a smaller scale, although specifications for a scaled down testing machine will be different. However, the ball screw material properties are independent of scaling and are applicable to any machine testing under ATSM 638-14.
Manufacturing of the ball screw, independent of scaling, will be accomplished utilizing an isotropic homogenous material. This will ensure constant properties independent of orientation and minimize potential point defects. The exact process of creating the ball screw will depend on the material chosen, however, they will likely be formed by rod drawing and then a subtractive machining process for the thread.
Important Properties:
Young’s Modulus
Fracture Toughness
Hardness
Corrosion Resistance
Material Selection
Material Selection
The critical constraint of our ball screw is the stiffness and the objective is the cost. In-order to achieve the material properties necessary to full-fill these constraints we modeled the ball screw as a Tie, minimizing cost, with a prescribed stiffness. This gave us the equation E/C_v,r. The materials that best fit this family of parallel lines were Cast Irons, Aluminum alloys, Steels and Silicon Carbide.
To ensure that our ball screw can handle continuous loading while complying with the defined constraints, we utilized the equation for displacement limited which gave a family of parallel lines with slope K_1c/E. In the chart we see that the materials that align with our needs are Aluminum, Zinc, Titanium, Nickel and Copper alloys. In addition Cast Irons and Steels fall well within our criteria.
For a high hardness and low wear-rate constant we utilize the equation K = k_a*H to determine what materials will allow us to maximize harness and minimize wear-rate. As can be seen in the chart the materials that best fit our needs fall in mainly in Aluminum alloys, various Carbon Steels and Tungsten Carbide ceramics.
Another important material property to consider is corrosion resistance. While we expect the machine to be used in a controlled climate i.e indoors, it is important to keep this property in-mind. From the aerated water section of the chart we can see that carbon steels have poor corrosion resistance while alloys and ceramics have much higher resistance to corrosion.
Through the use of the constraints and objective, we were able to create functions that could then be used to determine material indices that we then applied to Ashby Charts. This allowed us to easily identify many materials that may meet our projects needs. Through the use of multiple properties we were able to narrow our choices down to the three that best fit our needs, these can be seen highlighted on the above. It should be noted that while corrosion resistance is an important property, we do intend for this machine to be used in a climate controlled environment, thus corrosion resistance was weighted far less than the other properties.
Process Selection
Process Selection
One of the first things we need to identify is the required shape of our component. The ball screw will be solid and have a circular profile. This profile has many possible processes for metal as seen in the figure on the left.
Next, we need to ensure we choose a process that can cater to the mass of our component. For the ball screw, in large scale production, our processes are narrowed down to sand casting, forging, and conventional machining.
A crucial aspect of our ball screw is the tolerance of the finished thread. This will require a secondary finishing process to achieve.
Another critical quality of our ball screw is the surface roughness. We need a very smooth surface to ensure proper operation with the ball bearings. This also suggest a secondary finishing process will be required.
Finally, we must consider the cost as a function of batch size. With full scale production in-mind, and our material selection, this fully narrows down our process choices.
Through the use of the process charts we are able to narrow down viable processes to a primary and secondary process. It is evident that a single process will not be all of the constraints necessary for an ideal performing ball screw. Depending on final material choice, our primary process will either be heated extrusion or forging. Then to meet our constraints, we will utilize precision machining.
Page updated
Report abuse