Lifting Arm for SMRT Train Door Maintenance
Hello all!
We have built the aluminium framework that forms the base of our entire lifting system. The dimensions were informed by our prior calculations on Monday, 24th June, which involved finding the centre of gravity (CG) of our pneumatic unit.
<to be updated>
In case you forgot already, our actual pneumatic unit from the SMRT Tuas Depot was supposed to arrive yesterday (20th June). Unfortunately, a puncture tyre poked a hole in that plan.
Finally this afternoon, our friendly LTA engineer secured another van to deliver the actual, new pneumatic unit to the Making & Tinkering Lab for us to work on. (unit labelled with a big red circle in case you can’t see it amongst the organised mess of the bench)
As a comparison, here is the old unit (now sadly dumped into a corner of the MnT lab store)
Now, we were in for a slightly nasty surprise – there were a few key differences between the old and the new unit, which increases the obstacles for our clamping system and makes our life significantly harder.
Presence of wiring system and additional metal rod on the top side of the unit
Additional enlarged metal handle in middle of unit
Well, time to work on revising our clamping system so that we can overcome these challenges! Adios, for now.
Today, we plan to build a base for our lifting arm. But first, we will discover the CG of our arm. We require the CG of arm to ensure that the far edge of the base (pivot) is beyond the CG of arm such that our lifting arm will not topple over when mounted to the base.
Without the weight of actuators and worm gear, we derived that the base must be at least 1.62m long from the support tower to prevent the lifting arm from tipping over.
We plan to attach heavy-duty wheels to our lifting arm base, so that we can move our lifting arm and use it as a trolley too. This means that the point of contact that front wheels have with the ground must be at least 1.62m away. This may make our base quite large.
Alternatively, we consider having 2 3060 aluminum profiles to act as the support tower. If we approximate the weight of the actuators and worm gear as 2kg and 0.7kg respectively and include them in our moment calculations:
With a support tower reinforced with another 1.5m 3060 aluminium profile, actuators and worm gears, we calculated that the front wheels must be at least 1.13m away from the support tower to prevent the lifting arm from tipping over.
To increase the safety of our design, we can have base with front wheels about 1.25m away from support tower. The back wheels of the base can be about 0.15m away from our support tower. This effectively means that the distance between the front and back wheels of our lifting arm will be about 1.4m apart and the length of the base can be approximated to be about 1.5m long to accommodate for wheels placement.
The width of our base can be about double the width of our lifting arm at its widest point, with the front and back wheels placed just within the sides of the base. This should ensure that the CG of our lifting arm will be within our base, as well as provide enough breadth to place pneumatic units on the base.
Also, by balancing and adjusting the pneumatic unit on a 3060 profile, we approximated the Centre of Gravity of the pneumatic unit, as seen in the picture below. This centre of gravity will be kept in mind when we design the clamp system and the distribution of the clamps along the aluminium profile.
We also designed and 3D printed a mold for one of the attachments on the base, seen below:
Today, we hoped to finally get the pneumatic unit to work on. However, unfortunate circumstances and a nasty punctured tyre poked a hole in that plan. 🙁
Tomorrow, may be a better day.
On the bright side, we refined our clamping system by adding a long (adjustable) clamp that can reach to under some of the attachments to secure the unit in place while it is rotated upside down.
Also, we were linked up with our mentor, Jie Huang, who has some experience in Solidworks! He gave us some valuable advice – focus on a design that can lift our payload first, then look into stress/strain analysis. We will also be meeting him next week. 🙂
Finally, we procured some pillow blocks with 30mm bore diameter for use as rotating joints to connect two the two support aluminium profiles with the main lifting arm (rough sketch below). The pillow block allows 360 degree free rotation ability via the aluminium rod (30mm diameter) that fits through the 3 pillow blocks. To install control, actuators will be installed (as per our prototype designs) between the two support aluminium profile and the main lifting arm (not drawn below for simplicity).
After multiple redesigns and dimensions adjustments, we have finally achieved a design that can seamlessly pick up loads on the trolley base and lift the to a height of 2.9m for installation purposes, in the working space behind the LED screen and near the top of the train door.
This particular design, as crafted in SolidWorks, includes a set of 2 actuators between the support tower and first arm, a double actuator configuration between the first and second arm, worm gear between the second and third arm and ball joint between the third arm and clamping mechanism.
Between support tower and first arm:
We have chosen to have two 5000N actuators to support the torque of the lifting arms. These two actuators will be able to support the weight of the arms and pneumatic unit at the (impossible) worst case scenario of full arm extension with a safety factor of 2.5. Our calculations can be seen in the image below:
Between first arm and second arm:
We have chosen to use a double actuator configuration for the arms to be able to reach maximum extension and elevation.
The first actuator in this unique configuration will extend and push an intermediary arm sandwiched (with pillow block joint) between first and second arm. The first actuator will give the intermediary arm some additional height.
Then the second actuator, upon extension, leverages on this intermediary arm height to push the second arm higher. We envision that this double actuator configuration will allow use to achieve consecutive and parallel placements of first and second arms. This allows us to maximise height reached with minimum aluminium profile length used, via the Pythagoras theorem.
Between second arm and third arm:
We will use a worm gear between second arm to the third arm so the third arm can have the freedom to rotate 360 degrees. Since we estimated that the vertical working space behind the LED screen to be about 40cm, our third arm will be about 50cm to have sufficient clearance from the top of the door. This third arm will be able to rotate to pick up the pneumatic units on the trolley base and rotate to angle the pneumatic unit such that it can be slid into its holding place easily. This third arm will definitely add to the ease of use and versatility of our design.
Between third arm and clamping system:
We will use a ball joint, preferably one with a locking mechanism, for greater maneuverability of clamping system (which is attached to the unit). This enables the clamping system and unit to be easily moved and held in place, even as the angle of holding place changes slightly from door to door. This likely improves productivity and speed of installation/deinstallation as small adjustments of the unit can be made easily, without the need to shift the whole trolley front and back or side to side.
We considered some preliminary designs which use actuators (pictured below).
However, after doing some motion studies on our scaled paper model and some height calculations, we concluded that they were not the most suitable for our purposes of lifting a pneumatic unit from the ground and installing it at a height of 2.9m.
For the first and second design (from left), they were not able to pick the unit from the ground.
The first design did not have a long enough second arm and the actuator was placed such that the first arm is not able to go below about 45 degrees parallel to the ground.
For the second design, the placement of the second actuator between the first and second arm caused the arms to be unable to reach the pneumatic unit on the ground too.
For the third design, the design was unable to reach the required height of 2.9m due to the insufficient extension of second actuator. We will need to consider how to increase the reach of the second arm without resorting to cranes or longer actuator units (with lower and insufficient force outputs).
We bought some toggle clamps, pictured below, online a few days ago and they finally arrived!
These toggle clamps are quick-release and easy to use when clamping. This will enable the technician using this clamp to speedily clamp all of the clamps in the whole clamping system to secure the pneumatic unit for lifting or setting down. These clamps also come with holes such that they can be easily screwed onto aluminum profiles.
In addition, these toggle clamps come with non-slip rubbered surfaces, which promises a non-slip hold of loads up to 90kg. The length of its clamp is also adjustable, allowing us to customize the clamp lengths. The clamps can be removed and replaced with another clamp with has a longer adjustable length, if need be too. This enables us to have full customization over our clamp system and thus, will enable us to adjust our clamp system around the different obstacles on the unit, as seen in the photos below. It is clear why these toggle clamps are currently used in the industry to fashion clamping system – their strength, convenience and customizability is unparalleled.
Furthermore, we chose to work with aluminum profiles as they are strong and fully customizable. As seen in our annotated diagram below, the top bar can be adjusted to provide better clamping force and allow suitable clearance from obstacles at different heights from the base.
Currently, this prototype clamp mechanism was built with 2020 aluminium profiles, toggle clamps and 3D printed plastic L-shape brackets. While creating and tinkering with our prototype clamping mechanism, one of our 3D printed plastic L-shape brackets broke when we attempted to tighten the toggle clamp onto the base of our pneumatic unit.
We definitely needed to invest in stronger and higher quality L-shaped brackets. And so, we put in an order for metal L-shaped brackets for all of our profile sizes.
Can’t wait for them to arrive! 😀
On 5 Jun, our risk assessment finally got approved! YAY!!
To us, bastions of safety and wellbeing, this was a crowning achievement.
This meant the procurement of the safety paraphernalia. We went to look for safety boots online. We found some pretty-looking safety boots on Amazon. However, Tony, our experienced 5th to 10th member,said that safety boot sizes can be quite different from our normal shoe sizes, and recommended us to buy them in person. When we borrowed safety boots from SMRT for our Tuas Train Depot site visits, we also felt that the safety boot sizes are usually larger than normal shoe sizes. So we decided to buy them in person instead.
However, we faced lots of problems. Places selling cheaper safety shoes did not seem to have a retail or outlet store where we can try them out. We discovered that Timberland sold safety shoes, however, they were really pricey, at about $200 for a pair. A whooping $800 (about half of MnT budget) to buy shoes for all 4 of us.
hmmmm, how?
Also, we are reconsidering our first lifting design which has an extruding arm and worm gear. Worm gears, those in our price range at least, are too weak to lift the heavy load of the lifting arm and the unit, with a maximum torque of about 60-100Nm.
Hence, we decided to use linear actuators. They are much stronger and can exert a lot more force at about 3000-5000N. This meant that the linear actuators is more likely to be able to lift the lifting arm and the unit (to be substantiated with calculations soon).
The drawback of using linear actuators is the decrease in the rotational ability (directly translated to mobility in our first design). To counteract this, we will use a configuration of 2/3 arms connected with linear actuators. To preserve the high mobility of the clamping system, we will still use a ball socket joint between the lifting arm and the clamping system.
Based on the requirements and space restrictions presented by LTA and SMRT during the first depot visit, we went through a few iterations of possible crane design (as will be detailed in another post – insert hyperlink). As of 3 Jun, we sketched out our best conceived design on paper, detailed some of the parts necessary to built this model and thought through its working mechanism, pictured below:
We created this prototype model on SolidWorks too so we can use it to conduct motion studies in the future when we acquire more specific and precise dimensions, pictured below:
This prototype model features:
Hopeful we can one shot one kill with this design! 😀
Edit: Famous last words… :”
We were in talks with the LTA engineers to get a pneumatic unit to work with in lab. After a few days of waiting, they informed us that there was a pneumatic unit in the SMRT-NTU Smart Urban Rail Corporate Lab and linked us up with a researcher working in that lab! Finally, we could get our hands on the pneumatic unit and start visualizing and materializing our solution. 🙂
We went over to the SMRT-NTU Corp Lab with a trolley and brought the unit to the Making and Tinkering Lab. However, we soon realized that this pneumatic unit (which was on display in the SMRT-NTU Corp Lab) is not the same as the unit currently used in the trains. The pneumatic unit we got from SMRT-NTU Corp Lab is pictured below on the left and the pneumatic unit used for repair and replacement works in Tuas Depot is pictured on the right.
We won’t be able to get the actual unit anytime soon too.
This meant that we would be working with different dimensions and different attachments on the metal base of the pneumatic unit whilst brainstorming and creating our clamp system. Our crafted clamp system had to be adjustable to allow for these tweaks in dimension and attachments. Also, this probably meant that we will not be able to custom-build a grip that can hold onto any of the attachments.
We had to think of a way to work around this… Thinking caps on!
Also, since we will visit the Tuas Depot again tomorrow, we brainstormed some of the train and depot measurements to take: