问题 问答题

液压传动与机械传动比较,有哪些优缺点

答案

参考答案:与机械传动相比,液压传动的优点是:(1)可进行无级调速,调速方便且范围大。(2)运动比较平稳,反应快、冲击小、能高速起动、制动和换向。(3)控制调节元件操作简便,省力,容易实现自动化。(4)能自动防止过载,实现安全保护;液压元件能够自行润滑,寿命长。(5)在传递相同功率的前提下,液压传动装置体积小,重量轻,结构紧凑。
缺点是:(1)液压传动由于液体容易泄漏,因而对液压元件的制造精度要求较高。(2)液压传动容易泄漏所以难以保证严格的传动比。(3)在工作过程能量损失较大,系统效率较低,不宜作远距离传动。 (4)对油温敏感,不宜在很高和很低的温度下工作。(5)出现故障,不易查找原因。

阅读理解

In the long discussion of water on the Moon, a new study contradicts(与……相矛盾)) some recent reports that say the Moon had water at the time of its formation. A group of researchers reported in the journal Science that when the Moon was created, some 4.5 billion years ago, there was not much hydrogen(氢气) on it, and therefore no water.

The researchers surveyed and evaluated this by analyzing chlorine isotopes(氯同位素) found in lunar rock samples from Apollo missions. The range of chlorine isotopes in lunar samples was 25 times that found in samples from Earth.

If the Moon had significant levels of hydrogen, as Earth did, this range would have been far less, said Zachary D Sharp, a scientist in the Department of Earth and Planetary Sciences at the University of new Mexico and the study’s lead authour.

The chlorine would have stuck together with hydrogen, forming compounds like hydrogen chloride, and escaped from the Moon’s surface, he said. The abundance of chlorine indicates a lack of hydrogen and water.

“ The amount of water on the Moon was too low for life to possibly have existed there,” he said.

Most scientists believe the Moon was formed when a large object stuck Earth, breaking off a chunk(a vast piece) that has since orbited Earth.

On Earth, goes one theory, water was released as steam form molten basalts(玄武岩) over time, eventually forming bodies of water.

“An understading of whether the Moon was dry or wet will help us understand how water appeared on Earth,” Dr Sharp said.

小题1: A new study indicates that _____ on the moorn.

A.there is plenty of water

B.water is unlikely to exist

C.water existed in the past

D.there is a little water now小题2: The author would like to tell us ______ through the sixth paragraph.

A.why there is no water on the Moon

B.how the Moon was formed

C.when the Earth struck the Moon

D.where the Moon orbited小题3:  Which is NOT true according to the passage?

A. Zachary D. Sharp, a scientist of New Mexico was in charge of the new study.

B. Chlorine isotopes in lunar samples are 25 time larger than those on the Earth.

C. The more aboundant chloride, the shorter hydrogen and water.

D. The new study is benificial to understand how water appeared on Earth.

小题4: What can we learn from the passage?

A.There is enough water on the Earth.

B.Hydrogen chloride easily escaped from the Moon’s surface.

C.The Earth had significant levels of hydrogen.

D.Water on the Earth is directly from molten basalts.

多项选择题