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砝碼硬度計(jì)的特點(diǎn)|雙語(yǔ)
砝碼系統(tǒng)
早在1900年，當(dāng)使用金屬硬度測(cè)試作為一種質(zhì)量保證措施時(shí)（確切的鋼硬度）砝碼式硬度測(cè)試系統(tǒng)就被用于施加所需的試驗(yàn)力。原因很簡(jiǎn)單，砝碼式硬度測(cè)試系統(tǒng)便宜且易于制造，且能滿足當(dāng)時(shí)硬度測(cè)試的精度要求。
然而，砝碼系統(tǒng)的問(wèn)題是試驗(yàn)力必須通過(guò)杠桿結(jié)構(gòu)施加在試樣上，并利用大量的活動(dòng)部件得到最終的小壓痕。將砝碼系統(tǒng)產(chǎn)生的試驗(yàn)力轉(zhuǎn)移到小的金剛石壓頭或球壓頭頂部是這很難實(shí)現(xiàn)，如進(jìn)行洛氏HRC測(cè)試時(shí)需使用150kgf的主試驗(yàn)力。一個(gè)便攜設(shè)備很難滿足大尺寸和150kg的力，它要求設(shè)計(jì)者利用更小的砝碼和杠桿來(lái)放大試驗(yàn)力。
對(duì)于進(jìn)行3000kgf的布氏測(cè)量此問(wèn)題會(huì)更為嚴(yán)重，同時(shí)，對(duì)于小力值硬度計(jì)，如需要達(dá)到1gf是，也是一個(gè)挑戰(zhàn)。
杠桿需要支撐、導(dǎo)軌、軸承和其他產(chǎn)生摩擦的部件，這些部件會(huì)導(dǎo)致受力不穩(wěn)定。
雖然這些偏差的源頭可控，但系統(tǒng)中的任何摩擦最終都會(huì)產(chǎn)生不利影響，并在儀器的使用過(guò)程中越來(lái)越嚴(yán)重。同時(shí)，在砝碼系統(tǒng)中很難控制試驗(yàn)力，因?yàn)橐ㄟ^(guò)移動(dòng)砝碼得到所需的試驗(yàn)力，然而在沒(méi)有過(guò)載或震蕩的情況下是很難快速停止的。許多老的硬度計(jì)使用緩沖器（充油阻尼器）來(lái)控制，然而，緩沖器本身就存在如漏油，磨損，密封，溫度影響等問(wèn)題。因此，在后來(lái)的設(shè)計(jì)中，緩沖器被馬達(dá)取代了。
盡管一些阻尼缺陷消除了，但快速測(cè)量的要求則對(duì)馬達(dá)速度要求更嚴(yán)苛，所以，試驗(yàn)力過(guò)載和波動(dòng)問(wèn)題經(jīng)常出現(xiàn)。電動(dòng)砝碼式硬度計(jì)的工作速度慢，不適合生產(chǎn)。一般認(rèn)為，砝碼系統(tǒng)的試驗(yàn)力是保持一致的，但這種假設(shè)很難實(shí)現(xiàn)。
因?yàn)樽罱K施加的力是無(wú)法控制的，所以你可以稱之為開環(huán)力加載系統(tǒng)。
砝碼（開環(huán)）系統(tǒng)的優(yōu)點(diǎn)：
– 制造簡(jiǎn)單
– 造價(jià)低
– 模擬系統(tǒng)，無(wú)需供電
砝碼（開環(huán)）系統(tǒng)的缺點(diǎn)：
– 無(wú)法反饋實(shí)際的試驗(yàn)力
– 對(duì)溫度和環(huán)境較敏感
– 測(cè)量重復(fù)性和再現(xiàn)性差
– 機(jī)械加載系統(tǒng)復(fù)雜且不便維修
– 機(jī)械結(jié)構(gòu)易磨損
– 維護(hù)費(fèi)用相對(duì)較高
考慮到砝碼系統(tǒng)的缺點(diǎn)，INNOVATEST 軼諾所有的硬度計(jì)都不是砝碼系統(tǒng)，而是閉環(huán)傳感器控制系統(tǒng)。 閉環(huán)傳感器控制系統(tǒng)的優(yōu)勢(shì)，下篇文章進(jìn)行介紹。
DEAD WEIGHT SYSTEMS
Since early 1900 when the use of hardness testing of metals came as a measure to assure
quality (a certain hardness of steel) hardness testing instruments have used dead weights to
apply the required test forces.
The reason for this was fairly simple, because dead weights are relatively inexpensive and
easy to manufacture to the degree of accuracy required by commonly used test methods.
The problem of deadweights however is that the force must be applied to the test piece through a lever structure, with a large number of moving parts, to eventually a small indenter. Transferring the dead-weight force to the tip of a small diamond or ball indenter, for instance 150kgf main load used for a Rockwell HRC test, is difficult to accomplish. The large size and mass of a 150 kg weight would not fit in a handy instrument so it required designers to use smaller weights with levers to intensify the force to the desired levels.
The problem just gets bigger if the force increases to 3000kgf for Brinell measurements, but dead weight systems are also a challenge for designers for low force hardness testers going down to for instance 1gf.
Levers require pivots, guides, bearings and other friction-producing parts that cause force application instability.
Although it is possible to control these sources of error, any friction point in the system will eventually have a negative effect that slowly increases during the lifetime of the instrument. It is also not easy to control a dead weight application in a dead-weight system. Because the dead weight must be moved to apply the test force, stopping it quickly without overload and oscillation is problematic. Many older testers use dashpots (oil filled dampers) to control the application; however, these dashpots where source of problems on their own (oil leaking, wear & tear on the sealings, tempurature effects). In later designs dashpots where replaced by motors.
Although this eliminates some dashpot problems, the need to perform tests quickly makes motor speed critical, and as a result, force overshoot and oscillation are frequent problems. The motorized dead weight testers are also slow working machines, not realy suitable for production. It is assumed that dead weight originated forces remain consistent as each test is performed. But it remains to be an assumption, the reality shows plenty of problems.
Because there is no control on the eventually applied force, you could speak of an open loop, force application system;
Advantages of a dead weight (open loop) system:
Easy to manufacture
Low cost to manufacture
Analogue systems require no electric connection
Disadvantages of a dead weight (open loop) system:
No feedback on actual supplied test force
Temperature and environment sensitive
Low performance on Gauge repeatability and reproducibility
Complex & service sensitive mechanical force application system
Wear & tear on mechanics on a longer term
Relative high maintenance cost