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I am currently studying How excavators work? and came to know that "In excavators diesel engine is used to drive the hydraulic pump. The high pressure fluid from hydraulic pump is used to run the hydraulic motors which rotates the wheel." Is there any specific reason for using hydraulic motors instead of directly using diesel engine?

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Yes, because many or most excavators can 360 rotate so getting a classic gearbox/ prop shaft drive to the wherls or tracks would be challenging.

Then sorting the ability to do forwards with one side and reverse at the same time with the other is also difficult without hydraulics.

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    $\begingroup$ Additionally, the hydraulic system is already in place for managing the manipulators. $\endgroup$
    – fred_dot_u
    Nov 7, 2022 at 14:17
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A hydraulic motor has much greater torque density (torque vs size) compared to electric or diesel engines.

The hydraulic motor works well underwater, while an electric motor needs expensive (and heat retaining) waterproofing and a diesel engine needs air.

The hydraulic motor is simply more durable and efficient, despite any loss from converting from one power source to another.

https://info.texasfinaldrive.com/shop-talk-blog/hydraulic-motors-vs-electrical-motors-why-hydraulic-wins

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  • $\begingroup$ While most of that is true, the excavators still need a power source to run the hydraulic motor, so it will not be able to function under water if that power source is a combustion engine. Considering the whole powertrain, hydraulic motor is an extra "point of loss", it would be possible to construct a fully electric, or a combustion engine powertrain both of which would have better overall efficiency. Hydraulic system is much simplier, I'll give you that. Durability is simply a question of engineering, it's not dependent on the type of the system. $\endgroup$
    – Jpe61
    Nov 9, 2022 at 8:15
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    $\begingroup$ We can't do much better than that link, which clearly rebutts all but your first sentence. $\endgroup$
    – Rob
    Nov 9, 2022 at 8:47
  • $\begingroup$ Where exactly do the hydraulic motors get their power from? Just because a hydraulic motor manufacturer (reseller?) says on the internet that they are superior does not mean they are. Please have some source critisism. $\endgroup$
    – Jpe61
    Nov 9, 2022 at 9:46
  • $\begingroup$ Hydraulic motors are used in many types of equipment, Unbiased source: researchgate.net/publication/…. 2nd opinion: e-mj.com/features/… and agreement from an electric motor manufacturer: emotorsdirect.ca/knowledge-center/article/… - When the best answer is your own, you should write your own answer. $\endgroup$
    – Rob
    Nov 9, 2022 at 15:25
  • $\begingroup$ @SolarMike has the best anwer. Without hydraulic pressure, the torque density of a hydraulic motor is zero. What do you think produces the hydraulic pressure for hydraulic motors? $\endgroup$
    – Jpe61
    Nov 9, 2022 at 17:36

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