Daniel Duarte

Pump support structure

For a team project in Mechanical Analysis for Design, we developed a support structure for a pump-and-pulley assembly. The work connected structural analysis and material selection with component drawings and a physical prototype. The Group 28 assessment records a 117 lb applied load and 3.81 mm platform-end deflection.

Problem
Hold a pump and pulley on a platform at the position set by the test setup, carrying the applied load back into the mounting structure, with limits on deflection, height and budget and a score for low mass.
My contribution
With my two teammates, I worked on the load-path analysis, the stress and factor-of-safety derivations, material selection, the bill of materials, the prototype build and the load test. I also completed the separate material-selection assignment shown below.
Engineering decisions
We moved from a preliminary steel platform and arms to 6061 aluminum, for its strength, corrosion resistance, machinability and low density. We chose stainless steel for the pins, which needed more than 300 MPa yield.
Evidence
The assembly drawing and bill of materials, a component drawing, free-body and moment diagrams, a material-selection chart and the recorded prototype sheet.
Outcome
The prototype attached to the test setup and was recorded at a 117.0 lb applied load, with 0.15 in (3.81 mm) of deflection at the platform end.
Scope
These are single recorded team results. 117 lb is the load applied in the assessment, not a certified rating or a failure load. Our report set a 1 mm deflection limit, so the prototype didn't meet every target.
117.0 lb
Applied load recorded on the team's prototype sheet. Not a rating or a failure load.
3.81 mm
Platform-end deflection recorded at that load (0.15 in).
5 lb 8.2 oz
Recorded mass of all prototype components.
SOLIDWORKS assembly drawing titled ME321L Group 28 Pump Assembly, with isometric and side views of a platform carried by two diagonal arms from a mounting plate, and an eight-line bill of materials
The team's assembly drawing and bill of materials. The Drawn field in the title block reads DM. The title block's tolerance fields are blank, so this is a coursework drawing and not a production drawing package.

My work on the project

We shared the work across the team of three. My part included:

  • Drew free-body diagrams of the platform, arms and pins, and followed the load through each joint.
  • Derived the bending, axial, torsion, shear and pin-loading stresses, and the factor-of-safety and buckling expressions.
  • Estimated stress concentration factors at the pin holes.
  • Set material requirements and screened candidates with material-selection charts.
  • Helped put together the bill of materials against the $150 budget.
  • Helped build the prototype, mount it on the test setup, and run and record the load test.

Load path

Side view of the pump assembly with numbered marks on the platform, the diagonal arm, the three pinned joints, the pump and the pulley
Side view, cropped from the assembly drawing, with my numbered annotations.
  1. Platform. Carries the pump and pulley and is pinned to the mount at its back edge.
  2. Arm. One of two, in compression between the platform and the mount.
  3. Arm-to-mount pin. In direct shear.
  4. Arm-to-platform pin. In direct shear. The report expects the largest platform moment at this section.
  5. Platform-to-mount pin. In direct shear.
  6. Pump. Sits over the platform's four tapped holes.
  7. Pulley. The assessment loads the structure through the pump and pulley, and the report sets the platform length so the pulleys stay aligned.

The pump load and weight act near the platform's free end. The platform bends between that load, the arm pins and the pinned back edge, which also puts it in torsion, transverse shear and tension from the arms. The arms act as two-force members in compression, and the pins carry direct shear. The team expected arm BF to carry more than arm AE, and the highest platform stress near the pin holes.

Hand-drawn free-body diagram of the platform, with pump forces F1 plus F2 and moments at G, arm forces E and F at angle theta, pin reactions at C and D, and weights
Team free-body diagram of the platform. The arm forces E and F act at an angle θ.
Hand-drawn bending moment diagram along the platform, rising to a peak at the section of the arm pins and returning to zero at the back edge
Team moment diagram. The peak falls at the arm-pin section, which sets the critical bending location.
From the free-body diagrams to the stress checks

The team's report turns each component's load into a stress and a factor of safety. These are the symbolic forms, with Kt the stress concentration factor at the pin holes:

Platform: σmax = Kt 6Fl / (W t2),   δmax = 4FL3 / (W t3 E)

Arm: σmax = Kt F / (w0 t),   Pcr = π2E w0 t3 / (12 L2)

Pin: τmax = Kt 4F / (π d2)

Ductile materials were checked with maximum shear stress and brittle ones with Coulomb-Mohr. At the preliminary dimensions, the report estimates Kt of about 3 for the platform, 2.6 for the arms and 2.0 for the pins. The deflection expression treats the platform as a cantilever, a simplification of the pinned and braced structure.

Hand-drawn free-body diagrams of arms AE and BF, each with a force along the arm at the platform end and x and z reactions at the mount
Team free-body diagrams of the arms. Each has a single force along its length at the platform end.

Material selection

The report set requirements before choosing: at least 200 MPa yield and 70 GPa modulus for the platform and arms, more than 300 MPa yield for the pins, corrosion tolerance for freshwater exposure, machinability, and a $150 total budget.

Preliminary values are from the team report's first sizing, and selections are from the assembly drawing's bill of materials. The report's purchasing list, at $106.22, is an earlier version with 3/8 in pins.
PartPreliminaryDocumented assembly selection
PlatformSteel plate, 12 × 18 × 0.5 in6061 aluminum, 16 × 5.5 × 0.5 in
ArmsSteel bar, 15 × 1 × 0.25 in6061 aluminum, 15 × 0.9 × 0.25 in
Pins and rods3/8 in diameter pins304 stainless steel; the drawn holes are Ø0.250 in
Scatter chart of Young's modulus against density for candidate metals, with one highlighted and labeled carbon steel AISI 1020 as rolled
Team chart of modulus against density, from the report's platform section. Its only labeled material is carbon steel AISI 1020, a preliminary candidate. The report's text and the assembly select 6061.
Ashby chart of mass per unit of strength against flexural strength, with Aluminum 7255, Aluminum 7068 and Magnesium ZK60A labeled
My individual ME 321L assignment: a performance index, σ1/2/ρ, for a light platform in bending, and the resulting Ashby chart. It's a separate classroom screening, with different constraints from the team's selection.

Drawing

SOLIDWORKS drawing of the 6061 aluminum platform: 16 by 5.5 by 0.5 inches, four tapped holes for the pump, and two 0.25 inch through holes for the pins
The platform drawing shows the pump's four tapped holes and the two through holes for the pins. The Drawn field reads DM, and the tolerance fields are blank.

Recorded prototype results

QuantityRecorded result
Applied load117.0 lb
Platform-end deflection0.15 in / 3.81 mm
Prototype mass5 lb 8.2 oz
Assembled height13 in
Attachment to test setupSuccessful

The procedure raises the load in increments toward the project's specified maximum, then measures the deflection at the platform end. The sheet records one set of values. It doesn't record a failure load, repeated runs or fatigue behavior.

Our analysis report set a 1 mm limit for deflection at the platform's end, and the assessment gave full marks for 0 to 3 mm and the next band for 3.01 to 6 mm. The recorded 3.81 mm is above both, so the prototype carried the load but didn't meet the stiffness target.

Team

Group 28, with Dylan Matian and Miles Bradley.

Source files