# "Weight" a Minute

I don’t think I bamboozled anyone with my April Fools’ blog on Monday, so I’ll switch back to blogging in reality for another year. I’ve been speaking about  accumulators a lot lately, in blogs and on Twitter, so I thought I’d discuss the best – and least often used – version of hydraulic accumulator; the weight-loaded accumulator.

A weight-loaded accumulator is like a rod-up cylinder with a large mass attached, using the gravitational field of the Earth to pull the weight down, creating pressure in the fluid end of the cylinder. Weight-loaded accumulators are usually quite large (some downright humongous), and the larger the diameter of the piston, the bigger the weight atop the rod must be to create useable pressure.

The same simple f = m x A formula works here to calculate the pressure the accumulator gives, and other than calculating the volume it will provide using another simple formula (V=l x 3.14r2), there are no other complicated formulae like is required when making calculations for hydro-pneumatic accumulators. No expansion ratios, no gas laws, no pre-charge pressure ; just ask, “How much pressure do you want, and how long do you want it to last?”

A weight-loaded accumulator doesn’t have to factor in the compressibility of gases or springs, which change their fluid delivery characteristics based on how much they’re compressed. A fully filled bladder accumulator will provide diminishing flow rate as it empties itself and gas and work pressure equalize. A weight-loaded accumulator uses gravity to convert its energy, and the force of gravity doesn’t change measurably along its stroke length, allowing it to provide the same pressure and flow right until it bottoms out.

I can’t figure out why these accumulators aren’t more common. First of all, anyone can make one. Take a hydraulic cylinder, point it up and put some weight on it. I like the idea of a large bore, long stroke cylinder that can provide gallons upon gallons of flow when needed (mind you, you need to give it gallons upon gallons of flow to “recharge”). This system would provide benefits in many applications. Perhaps in a place with intermittent electricity and low-demand hydraulics, where you store enough energy for an entire day of usage and the pressure would remain the same until the accumulator is empty.

They could also be used in circumstance where low noise is a must, but the extra space for a remotely-located power unit isn’t an option. A high power pump/motor unit could charge the system in minutes and then provide an hour worth of energy, like a high volume air compressor receiver. Please respond in the comments section if you’ve seen or applied a weight-loaded accumulator recently, because I’d love to hear about it.

#### Discuss this Blog Entry 3

Marcus Driessen (not verified)
on Apr 6, 2013

Someone i know has built a hydraulic to hybride system with accumulators on a truck. I know it saves up to 20% fuel, engine shuts off when loading something by a crane etc..

Or are you 'only' interested in weight-loaded accumulators? :)

eg Twitter: @DHSbv picture 1, 9, 10

Kind regards!

on May 13, 2013

Hi Marcus! Sorry for the late reply. I am interested in everything related to hydraulics. Hydraulic hybrids are fantastic, and for heavy equipment, they actually beat electric hybrids for power density (they can provide massive torque for a short period, where electric hybrids can provide low to moderate torque for long periods).

Did your friend do anything themselves?

Josh

on Sep 12, 2013

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