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Hydraulic systems seem to have become ubiquitous. From huge cranes and excavators to disk brakes to door hinges. Basic requirement everywhere is the need to multiply force; trading greater force for lesser movement, the latter being compensated by use of electric motors or petrol/diesel engines.

My question is, when the same mechanical advantage can be obtained by using a couple of gears meshed together, why use a system that is costlier, complicated and messier ?

Advantages of hydraulics: Can transfer force through long curved paths, which otherwise would be tedious to accomplish through mechanical linkages.

Disadvantages I envision:

  1. Use of fluids with the optimal thermal and chemical properties for the job might increase the cost.

  2. Fluids are leaky, leakproof design adds to the cost.

  3. Piston must be of precise dimensions without any gaps; again more investment in R&D.

  4. Needs additional circuitry of valves, reservoir, pumping and recirculation.

So, all in all, they are increasingly used in smaller and trivial applications just because customers want it because it sounds cool to use a hydraulic device that forces fluid through a cylinder or are there real advantages ? Please explain the real benefits.

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  • $\begingroup$ One clear difference - the hydraulic cylinder applies a persistent pressure and dissipates the energy easily; the pressure delivered by the gear system is impulsive in nature and energy dissipation requires mechanical means. $\endgroup$
    – r13
    Dec 31, 2021 at 22:18

6 Answers 6

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  1. Power density. Modern hydraulic systems are way better compared to ropes and pulleys, gears, electromagnetic systems or levers.

  2. Freedom of geometry. Put a cylinder somewhere, get a rubber hose to it, done. Okay, two hoses in the general case. Almost like electricity.

  3. Hydraulic fluid also acts as a coolant and lubricant, no separate cooling and lubrication systems needed. Power-dense systems usually require both lubrication and cooling, so you have to deal with leaks anyway.

  4. "Force density". Imagine exerting great forces with meshed gears. This generally means extreme pressures in the contact spots of gears and bearings (and gears of great diameter). Hydraulics? Apply moderate (engineering-friendly) pressure, done.

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  • $\begingroup$ point 1: vehicle transmissions too handle huge amounts of power within relatively small space, is this space larger than the volume of the final cylinder in a hydraulic system ? point 2: agreed. but i find hydraulics even in applications where geometry is small well defined, case of vehicle disk brake. point 3: agreed point 4: extreme pressures need to be handled at some point in the path, even in hydraulic cylinder, the final cylinder walls and piston need to be thick enough to handle huge pressures. $\endgroup$ Dec 31, 2021 at 14:53
  • $\begingroup$ @SiddhAarth you mean the traditional automatic transmission? Other than that, gears are more efficient than hydraulics. This is one of the reasons why manual transmission is more efficient than automatic. $\endgroup$
    – fraxinus
    Dec 31, 2021 at 19:52
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Hydraulics can be used more naturally with engines for servo-like motions (try reversing direction or holding position on a engine driven gear box).

Piston must be of precise dimensions without any gaps; again more investment in R&D.

Gears need precision too so its not really a disadvantage of hydraulics.

won't a couple of gears do the job?

Except a couple of gears would not do the job...

My question is, when the same mechanical advantage can be obtained by using a couple of gears meshed together,

This is not an fair comparison. Think about electric motor speeds and the reduction possible with only a couple of reasonably sized gears. Still far too fast and far too low torque.

why use a system that is costlier, complicated and messier ?

Gearboxes are not trivial devices either. They fit into a small box and don't have lots of external parts so aren't spread around like hydraulic systems, but that is not the same as being simpler.

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  • $\begingroup$ I think , we can separate the source of power from the force multiplication device. Power source can either be electric or diesel or even manual(like car jacks) irrespective of the end device. I am just curious about the latter, which does this: Given a force F, increase it to Fx. "couple of gears" was just a figurative phrase used by me. To acheive a force multiplication of 1000, you need 3 gear meshes, each gear 10 times the size of other. Vehicle gearboxes do other stuff like switching gear ratios, syncro etc. $\endgroup$ Dec 31, 2021 at 15:09
  • $\begingroup$ @SiddhAarth Imagine the gear size for 10:1 taking into account required forces to not strip the smaller gear. 10:1 is a LOT. You can't separate prime mover from transmission if you want to talk about specific applications. To me, it doesn't matter if "a couple of gears" was an offhand phrase. It is indicative of a dismissive thought process about the difficulties of gear boxes. We try really hard to eliminate gears in our machines. Gearboxes tend to be one of those things that look simple because they present a neat, tidy, end package...like computer chips. $\endgroup$
    – DKNguyen
    Dec 31, 2021 at 15:21
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Fraxinus answer has covered many valid points (i.e. Power density, Freedom of geometry and cooling/lubrication).

Other significant reasons are:

  • Hydraulic systems are simple, safe and economical because they use fewer moving parts compared to mechanical and electrical systems. This also makes them easier to maintain. Maintenance is also easier, because in a hydraulic system, usually the parts that are being replaced/topped up is the fluid and the filters, even after extensive use. Gears on the other hand with long use they can wear out and need replace (and also calibration when assembled back).
  • Fire safety friendly: Hydraulic systems are safe to use in chemical plants and mines because they do not cause sparks.
  • Leak is easy to spot: problems in hydraulic system are easier to spot (actually some non destructive techniques use liquids to identify cracks)
  • Relief/check valves: Properly designed hydraulic system can have check/relief valves that protect the system from power failures and excessive pressures. Similar systems in gears are not so ''naturally' implemented.
  • Sudden reversing without disengaging: A gearbox in order to change gear or shift gear needs to decouple the motor from the gearbox and then shift gear. In the case of a hydraulic system without disegnaging, the reversing can be performed by change the state of a hydraulic valve.

Also a difference (not so much an advantage) is that the hydraulic work more intuitively as linear actuators, while gears are for rotating systems (although both of them can perform linear and rotary applications).

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Obvious reason it that the hydraulic system can be in many separate locations and provide power.

Think of a JCB arm with 2 or 3 joints and trying to connect gears with shafts for that strength and flexibilty. A couple of hoses and job done.

Then factor in the costs and the mass involved.

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  • $\begingroup$ "many separate locations" - yes understood, in cases where the source and application of power generation are far apart it makes sense, just connect a hose, as the answer of fraxinus says. But in the JCB example, i don't get it. JCB arms are anyways fixed straight lines, right ? you can just connect an axle like the rear wheel drive systems of vehicles. Then, the "cost and mass" - frankly, could you elaborate it ? Intuitively, the mechanical system seems to cost less, as it requires fewer and simple components. $\endgroup$ Dec 31, 2021 at 14:43
  • $\begingroup$ Not sure what JCB’s you have then. $\endgroup$
    – Solar Mike
    Dec 31, 2021 at 14:54
  • $\begingroup$ I mean, the individual arms, not the whole arm $\endgroup$ Dec 31, 2021 at 14:59
  • $\begingroup$ "you can just connect an axle like the rear wheel drive systems of vehicle" "Just"? That sounds way more complicated to me. A car doesn't have multiple joints at extreme angles. Only one rod with slight angles on each end. For a JCB, each distal arm segment needs a rods running down all less distal arms. So increasingly more torque rods running down the arm segments as they become less distal and then you need a way to transfer it around the joints. $\endgroup$
    – DKNguyen
    Dec 31, 2021 at 15:05
  • $\begingroup$ @SiddhAarth it’s what you mean that is not clear - why do you think I was talking about a short section of a jcb arm? Or, why would you limit it to just a short section? $\endgroup$
    – Solar Mike
    Dec 31, 2021 at 15:24
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"Can transfer force through long curved paths" - this part can in principle be done with cables, which is the technology that was used for construction equipment before hydraulics.

Historically, before hydraulics, cables were used to transmit force, rather than direct use of gears to generate linear motion. Gears would still be used to get variable speed though.

cable-steam-shovel

image: random google search for "cable steam shovel"

Another advantage of hydraulics was thus to eliminate the cables and pulleys. To get both strong force and decent range-of-motion, a cable-and-pulley system would needed a long (i.e. range of motion multiplied by the reduction ratio) run of suitably heavy cable, so this became a meaningful benefit at larger scales and loads. Being able to both push and pull was also easier to implement.

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I have rarely seen an equipment, a machine, using only hydrolic sytems or gears to transfer power and move levers and buckets, etc.

They use all systems in combination to get the optimal performance.

Many cranes, boldozers, tow trucks have part of their mechanism's articulation by gears and other parts by hydolic systems.

large earth moving machines benfit most by using hydrolics systems deliver the power near the end of a boom, or arm, where it is most efficient as oppsed to a gear delivering the power at the hinge or elbow which woul be suffering the greatest moments.

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