Platform Lift Design
Table of Contents
|MSD I & II||MSD I||MSD II|
Phase 1: Platform Lift Design
During Week 1 and Week 2 of the spring semester, design activities focused on the concepts that were selected during MSD I. Design work on the platform mattress lift was divided into two main focus areas: the platform itself, and the pulley-cable-winch mechanism for raising the platform.
Relevant DocumentsThe following documents are relevant to this phase of the project:
- Brief presentation on the platform lift concept, given to Autodesk representatives: Autodesk Presentation.pptx
Design PhilosophySeveral key choices underlaid the platform design. First was the decision to utilize a bolted construction, rather than a welded construction. This decision was driven by the failure of the cots built for St. Joseph's by the previous MSD team. Those cots were constructed of welded aluminum tubes. When the cots broke, the staff at St. Joseph's were unable to repair them. Therefore, we elected to design a platform that utilized screws as fasteners, rather than welds.
Material SelectionHaving made this decision, we then made a decision regarding construction materials. The inputs to this decision were our MSD II timetable, the decision to go with bolted construction, and the safety concerns we had with suspending mattresses in the air. Recognizing that design activities would occupy a significant portion of MSD II, we wanted to choose a material that would require a minimum of processing operations to produce a final part. We wanted to minimize the weight of the platform (while still providing adequate strength to support the weight of the mattresses), so aluminum became a preferred material over steel. We selected aluminum T-slotted framing as a material rather than tubular profiles, because tubular profiles would require us to drill many holes in order to realize a bolted construction. T-slotted framing, however, is designed with bolted construction in mind.
We therefore elected to design a frame for the platform of T-slotted framing, with sheet aluminum providing a smooth surface for the mattresses to rest on. The frame we designed used 1.5-inch square T-slotted framing as the primary structural members. We chose to use 2-slot framing with the slots on opposing sides, because we felt that this provided fewer hiding places for bed bugs. (See figure below)
Aluminum T-slotted framing profile used in the platform design. The cross-sectional dimensions of the profile are 1.5in X 1.5in.
The joints between the frame structural members were secured using 90-degree angles and T-brackets. These brackets are off-the-shelf components which are specifically designed for compatibility with 1.5in-square aluminum T-slotted framing. This design choice, again, will save us time during the build process because it will reduce the number of operations required to produce final parts. These parts are 4.5in long, 4.5in wide, 0.25in thick, and composed of aluminum. They are pre-drilled with through-holes for 5/16"-18 screws.
The only modification required to these parts is the addition of the sixth, "central" hole to the 90-degree angle bracket. This hole will provide an attachment point for the lifting cables.
Modified 90-degree angle bracket. Note that a sixth hole has been added in the center of the part, near the diagonal edge.
Structural MembersTwo designs for the frame immediately suggested themselves: two long members and three cross members, or 3 long members and two cross members. The three-long, two-short design was selected because long beams incur larger deflections and higher stresses than shorter beams. Using three long members therefore stiffens the frame along its weakest axis. In addition, this configuration leaves the aluminum sheet unsupported over 2ft-by-8ft area, while the two-long, three-short configuration leaves the aluminum sheet unsupported over a 4ft-by-4ft area. The 2ft-by-8ft area should be stiffer because bending across a 2ft length is eight times stiffer than bending across a 4ft length. The frame consisted of three 93in-long members and two 48in cross members as shown below. The overall length and width of the platform are 96in and 48in, respectively.
Positioning of frame structural members. Enlargements of a corner (bottom left) and "T" (top right) show how the long members are positioned relative to the short members.
Brackets and FastenersThe structural members are secured at each joint using the 90-degree angle brackets and T-brackets shown above. Off-the-shelf fasteners (see below) and 5/16"-18 screws are used to secure the bracket to the framing. The fasteners are inserted into the slots in the framing prior to assembly. The screws are passed downward through the holes in the brackets and into the threaded holes in the fasteners. When the screws are tightened, they clamp the bracket to the framing.
The brackets were attached to the bottom side of the structural members, rather than the tops. Force from the lift cables will be transferred to the eye bolt, which will in turn transfer the force to the brackets. If the brackets were installed on top of the structural members, the 5/16"-18 screws would be responsible for transferring the force to the framing fasteners, which would then transfer the load to the frame. This would most likely work. However, it was deemed safer to position the brackets under the structural members, where they can transfer the force directly to the structural members, thus eliminating any possibility of failure due to thread pull-out.
90-degree angle and T-brackets are positioned below the structural members to ensure that forces are transferred directly from the brackets to the structural members.
Lift PointsThe lift cables attach to eye bolts on the platform. One eye bolt is installed in each of the 90-degree angle brackets on the corners of the platform, as shown below. The eye bolts pass through the holes that were added to the brackets for that purpose.
Installation of the eye bolts in the 90-degree angle brackets in the corners of the frame. The installation is shown here from the bottom, and the aluminum sheet that the mattresses sit on has been hidden for clarity.
Mattress SurfaceThe eye bolts also pass through the aluminum sheet that the mattresses sit on. Therefore, holes must be drilled in the aluminum sheet to allow installation of the eye bolts. The aluminum sheet is attached to the platform using the same fasteners (albeit single-screw fasteners, not double-screw fasteners) as the brackets. Holes must be drilled in the aluminum sheet around the edges where the screws will pass through it. We have elected not to fasten the sheet to the central long member, because we are concerned that dragging the mattresses across the heads of the screws may cause rips or tears in the mattress covers. A top view of the platform is shown below.
Platform top view. Note the eye bolts in the corners, and the lack of fasteners in the central long structural member.
Floor ProtectionTo prevent the metal frame from damaging the floor of the shelter, rubber bumpers are installed as "feet" at each corner, as shown below.
Rubber bumper installed in the corner of the platform to prevent the platform from scratching or chipping the floor.
Cable RoutingWe looked at several options for lift mechanisms. We decided that it would be best to have the winch or motor at floor level, to make it easier to repair. Four cables will run from the platform vertically to the ceiling. At the ceiling, they will be redirected by pulleys and run to a central pulley. A single cable will attach to the four platform cables, run across the ceiling to the wall, where another pulley will direct it down the wall to the winch. See the schematic below.
Lift WinchFor simplicity, we elected to use a winch rather than a motor. The team surveyed a number of different styles of winches. The results of this survey can be found in P17431_BrakeWinchBenchmarking.docx. The winch we selected is a brake winch, which will take up some of the load when the mattresses are being lowered, so that the person operating the lift will not have to resist the whole load themselves. The winch can be operated using a bit for a standard power drill, which is advantageous because it reduces the effort required from the person operating the lift and speeds up the lifting of the mattresses.
Input from Subject Matter Expert (SME)
Comments ReceivedOn February 7, we met with Dr. Abdullah Al Faruque, a professor in RIT's Civil Engineering Technology department, to discuss the platform lift idea. In particular, we were concerned with the structure of the ceiling of the hospitality room at St. Joseph's. We have no way to determine the construction of the ceiling, and therefore no way to ensure that the ceiling will be able to support the lift and mattresses. We hoped that Dr. Faruque might be able to give us some helpful suggestions for determining the strength of the ceiling.
After explaining our concept and the difficulty we have in determining the strength of the ceiling, Dr. Faruque had two insights for us. First, that the loading we are applying to the ceiling (about six pounds per square foot) is very low by typical standards. However, lacking any information about the construction of the ceiling, he told us that he would strongly discourage attaching any components of our system to the ceiling. Instead, he suggested an alternate method for supporting the lifts that would utilize the walls for support, rather than the ceiling.
He suggested attaching a support, perhaps as simple as a length of 2x4 lumber, to opposing walls of the hospitality room. On top of these supports would rest metal beams that spanned the width of the room. The lifts could then be suspended from these beams. (See sketch below.)
Sketch of the alternate method for suspending the platform, suggested by Dr. Abdullah al Faruque. A view of the system from above (top sketch) and in the room (bottom sketch) are shown.
Post-Meeting Discussion and ConclusionsAfter some discussion of the possibility of implementing this system, we decided that it, too, is infeasible. The walls of St. Joseph's are constructed of brick, and it would therefore be difficult to install the 2x4 supports. In addition, we would be unable to test the supports without causing damage to the walls of St. Joseph's, which we do not want to do. The addition of beams at the ceiling level will also decrease the amount of head space below the platform, because the platform will not be able to raise all the way to the ceiling.
As a result, we abandoned the platform lift idea and went back to the drawing board to develop and explore other systems-level concepts.