Friction calculation

Discussion in 'Engineers' Talk' started by Lawrie Spark, Jun 23, 2022 at 10:36 AM.

  1. Lawrie Spark

    Lawrie Spark Member

    I've put this in the Engineer's talk as I couldn't see a better place... it's sort of quite a technical question.

    I have a hand built bed where the contact with the floor is via 2 strips of 36mm ply 1.2m wide at the bed head and at the bed foot. The bed needs occasionally to be slid from side to side 200mm - ie along the length of the ply.

    I had originally planned to put castors on it but I had some appliance rollers (the ones used for under washing machines) so used them. They work fine BUT they mark the floor with their internal grease.

    I was wondering about using felt furniture pads instead - the floor is painted timber laminate so quite slippery.

    The question is, is it better to put those pads on all of the surface that touches the floor or would it be better to use fewer pads?

    I guess there is an optimum balance related to weight and contact surface area. Does anyone have any thoughts?
  2. rogerk101

    rogerk101 Screwfix Select

    I've solved the same issue by gluing carpet bits to the underside of the bed's feet. The carpet bits are just a tad smaller than the feet so you can't see them without really looking for them.
    The bed now slides easily on the oiled engineered oak flooring but there is still sufficient friction such that the bed doesn't move unless you want it to.
  3. Lawrie Spark

    Lawrie Spark Member

    That's useful thanks - and I guess carpet is less likely to wear out than those pads.

    I have more potential contact area with the floor than just legs so have the option to cover all or part but I can't get my head around whether a bigger surface area will bring about more friction or less....
  4. Abbadon2001

    Abbadon2001 Active Member

    It will depend on the friction coefficient of the "pads" under different loads, more pads, less load per pad, and there will probably be a sweet spot where you meet the best compromise of pad area contacing floor vs the friction per unit area based on the load per square m. probably best to use trial and error :D
    Lawrie Spark and rogerk101 like this.
  5. stevie22

    stevie22 Screwfix Select

    You're overthinking: shove a bit on each corner say 100 long and 36 wide and see what happens.

    My grey cells are fading fast but I suspect that force/area gives contact pressure. Factor in friction to give effectively a breakaway shear stress (when it starts to slide). More pad area means lower contact pressure hence lower shear but the greater area comes back to the same potential force (ie how hard you got t o push).

    I actually have several blocks made up for shifting furniture on laminate floors. They are about 100 square of 18 ply and have a further 12 ply piece on top with a hole to locate furniture leg. Carpet piece is about 15 wider all round which seems to work slightly better than matching the ply
    Lawrie Spark likes this.

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