Episode 12: Concept of an Elevator

Motivation & Idea

In the article 1 Scale Layout one end of the line was executed as a fiddleyard. This is due to the fact that there is no room for a helix. The same applies to a staging yard then… unfortunately. Of course, it would be nicer to be able to use a staging yard anyway. But what if you could replace the helix with something more space-saving… For example, an elevator! Here are a few considerations.

Requirements

In general, here are the characteristics that the elevator should fulfil:

Specification sheet

  • Raise or lower a load (train) from one height level to another; height difference: approx. 400 mm
  • Load: up to 20kg (with heavy locomotive, heavy-duty car and loading it can be a lot 🙂
  • derailment safe positioning
  • one stroke shouldn’t take too long (you shouldn’t have to wait 5 minutes until another train comes… although it would be pretty realistic 🙂
  • quiet operation
  • DCC control (the lift should be controlled by Rocrail in the future)
  • Status message via feedback sensors (the lift should be able to report its status to Rocrail so that it can potentially be integrated into the automatic train control)

Safety

If you take the above facts together, it becomes clear that there is also a certain risk potential. A very powerful drive is required; in addition, there are various shearing and clamping points due to the design. Therefore, the following safety requirements must be observed:

  • Switch-off on overload (monitoring of the controller output)
  • Manual control (just for the case it must be possible to intervene)
  • Emergency stop
  • Intrinsically safe design (e.g. the spindles must not move into the mechanical end position => the construction would be damaged)

Let’s summarize the above points. The following specifications can be formulated for a functional model:

Functional Requirements

  • 2 trapezoidal threaded spindles:
    • great power development with a lot of reserve
    • quiet propulsion (no tooth engagement)
    • Self-locking (! In the event of a power failure, everything should remain still even with heavy loads !)
  • 2-3 guide bearings for clean running in the transverse axis; lateral guidance of the spindles
  • Repeat accuracy in the vertical axis: Should be at most 0.3 mm during positioning. The height difference at the rail joint should be small. A train should hardly roll over it audibly
  • Repeat accuracy in the transverse axis: Should be at most 0.5 mm. The lateral offset at the joint must be so small that there is no risk of derailment.
  • Travel speed of the carriage approx. 20 mm/s. A stroke lasts then approx. 20 seconds
  • Central DC motor with sufficient power: 12V motor with max. 200 W. It must provide sufficient reserves due to friction losses in the spindles
  • Power distribution via toothed belt drives
    • Very quiet
    • Synchronous running of both spindles => the slide must always remain horizontal
  • Silent motor control with 20kHz (“simple” control with 0,1 – 1kHz are noisy, which I find very disturbing)
  • Measures for safety shutdown:
    • software limit switches
    • hardware limit switches
    • finely adjustable position limit switches
    • Position sensor (also functions as speed sensor)

Concept

With these given mechanics this is the outcome:

  • A movable carriage: a T-beam made of joinery wood should be stable enough. The length of the movable track section is set to be 1420 mm in accordance with the maximum train length
  • 2 linear guides: Drawer guides from the DIY store may be used here. Extension length: 500 mm
  • 2 trapezoidal threaded spindles: not too thin (buckling length), not too thick (expensive & too large)
  • 1 drive motor (wiper motor)
  • 2 belt drives with tension pulleys
  • Various sensors (mechanical push buttons, multiturn potentiometers)
  • Wooden basic structure

Functional Model

The concept sketch can already be seen in the video (sorry for the poor image quality of the functional model itself):

Left and above (thick lines) both tracks are shown. The carriage is drawn in the upper position. It is guided by two lateral linear bearings (blue). The actual load lies on two threaded spindles (orange). The threaded spindles are driven by a centrally arranged DC motor. By using toothed belts, the spindles always run synchronously.

Summary

The first positioning movements of the elevator functional model shown are promising. In any case, there is enough power!



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