# Calculate mobile base weight to support adjustable arm

I want to calculate how heavy a mobile hydraulic post table base must be to support an adjustable monitor arm and monitor.

Monitor (iMac) Specs:

• Height: 20.3 inches (51.6 cm)
• Width: 25.6 inches (65.0 cm)
• Weight: 30 pounds (9.54 kg)
• Ship Weight: 120.0 lbs
• Platform Size: 16 x 16 in
• Raised Height: 49 in
• Lowered Height: 31 in
• Product Weight: 17.5 lbs (8 kg)
• 20 inches (51 cm) of vertical adjustment
• 62 inches (157 cm) of side-to-side motion
• Screen ≤ 46"
• Capacity 14–30 lbs
• Lift 20"
• Tilt 80°, Pan 360°, Rotation 90˚

The one capability that I think could cause the post table to collapse is the 62 inches (157 cm) of side-to-side motion. I've included a drawing.

• Are the arm and base compatible? I see the base has some sort of socket or something at the center, but you seem to want to clamp the arm to the side of the base instead. – Wasabi May 20 '16 at 12:29
• No these are two totally different products, so I'm trying to hack it. – Philip Michael May 20 '16 at 15:56

To calculate this, we need to know what tipping moment your table will suffer. This is pretty simple.

With the arm totally extended, it is $31''$ long (half of $62''$). You seem to be clamping the arm to the table, so I'm going to assume that the distance from the screen at the tip of the arm to the center of the table is equal to $31 + \frac{16}{2} = 39''$. The pivot point when tipping, however, will be one of the wheels, so we can subtract the distance from the post to one of the wheels. This isn't given, so I'm going to assume that distance is around $5''$, which gives us a total arm of $34''$.

So, now we have to calculate the tipping moment, considering the weight of the screen and of the arm.

$$M = 30\cdot34 + 17.5\cdot\left(3 + \dfrac{31}{2}\right) = 1344\text{ lb-in}$$

Now, this is already resisted by the weight of the post table (120 lb1), which can be considered concentrated at the post, generating therefore a moment equal to

$$M = 120 \cdot 5 = 600\text{ lb-in}$$

You therefore have here an excess of $1344-600 = 744\text{ lb-in}$ which you'll need to counter with additional weights. We can find the necessary weight by distributing it uniformly on the base, which gives us:

$$W = \dfrac{744}{5} = 148.8\text{ lb}$$

This is however only the nominal weight that's necessary. I would put in quite a bit extra weight to be sure the structure will hold in unexpected situations (someone bumping against the screen or some such).

The table states it can support 300 lb, so this is still acceptable: $$148.8 + 17.5 + 30 = 196.3 < 300\text{ lb}$$ Hell, I personally don't see why not simply put 250 lb of weight, just as a safety precaution.

You also need to check the clamping of the arm to the table. For this, however, we need more information regarding the table (thickness, material) and of the clamp.

1 This is actually the ship weight, which means the table will be lighter. Get the correct weight and repeat these calculations. Make sure to also check the actual distance from the post to the wheels, which I assumed here at $5''$.

• I've been re-reading your answer to make sure I know how to calculate it with the correct weight/other measurements. Thank you so much! – Philip Michael May 20 '16 at 15:57
• If this question was useful, please vote it up, I'm new to stackexchange and I'd like more abilities on the website. Thanks! – Philip Michael May 20 '16 at 15:59
• @PhilipMichael, please see my edit. Another point of failure in the structure is the clamping of the table and arm, but to solve that we need information. – Wasabi May 20 '16 at 16:30
• I like the 250 lbs of weight just as a precaution, but hopefully it's overkill. From your comments I'm going to do a Grommet Mount, where you just drill a hole in the table and screw it in through the bottom instead of using a clamp. I think the clamp being off to the side and not in the center is throwing everything off – Philip Michael May 20 '16 at 17:34
• The specs for the table are terrible online, but here's more about the arm – Philip Michael May 20 '16 at 17:36