Machinery Safety 101

Force and injury — How hard is too hard? ISO/TR 21260 will help

This entry is part 5 of 5 in the series Hier­archy of Controls

Update 2020-10-31: The work on ISO/TR 21260 is set to restart with a meet­ing in Dec-20. The seed doc­u­ment for the work is the last draft pre­pared dur­ing the ori­gin­al devel­op­ment cycle, which is good news as los­ing that pro­gress would have been a bad thing. More updates to fol­low when I can tell you more about the pro­gress in the Work­ing Group. 

Update 2020-10-02: TL;DR – Work on ISO/TR 21260 was stopped for a peri­od of time because the pro­ject had exceeded the devel­op­ment timeline allowed by ISO. In March of 2020, a com­mu­nic­a­tion indic­ated that a decision would be made at the next ISO/TC 199 Plen­ary meet­ing, sched­uled for the fall of 2020. With the glob­al pan­dem­ic, a decision was made to change ISO 21260 from an Inter­na­tion­al stand­ard to a Tech­nic­al Report (TR). Accord­ing to the ISO/IEC Dir­ect­ives, Part 2, a Tech­nic­al Report is defined as:

Tech­nic­al Report

doc­u­ment (3.1.1) pub­lished by ISO or IEC con­tain­ing col­lec­ted data of a dif­fer­ent kind from that nor­mally pub­lished as an Inter­na­tion­al Stand­ard (3.1.4) or Tech­nic­al Spe­cific­a­tion (3.1.5)

Note 1 to entry: Such data may include, for example, data obtained from a sur­vey car­ried out among the nation­al bod­ies, data on work in oth­er inter­na­tion­al organ­iz­a­tions or data on the “state of the art (3.4)” in rela­tion to stand­ards of nation­al bod­ies on a par­tic­u­lar subject.

Note 2 to entry: Pri­or to mid-1999, Tech­nic­al Reports were des­ig­nated as Tech­nic­al Reports of type 3.

ISO/IEC Dir­ect­ives, Part 2

This change allows devel­op­ment to con­tin­ue without the loss of the work done in the pro­ject. Pub­lic­a­tion of ISO/TR 21260 is now anti­cip­ated at the end of April 2023, although it is pos­sible that it could be pub­lished before then.

This post was updated 2020-06-10 with new inform­a­tion on ISO/TR 21260.

For any­one involved in risk assess­ment and con­trol, there are always ques­tions regard­ing the amount of force it takes to injure a per­son. As a remind­er, force is a vec­tor quant­ity that has mag­nitude and dir­ec­tion and, if unop­posed, will cause an object to change its velo­city and direction. 

As soon as we decided that products that hurt people were not OK, engin­eers wanted to know what the lim­its were on forces applied to people. In the US mil­it­ary, these ques­tions applied to air­men fly­ing mil­it­ary fight­ers, and sub­mar­iners fight­ing below the sur­face. Divers that were used for tac­tic­al actions and for sal­vage, repair and recov­ery mis­sions needed to know how deep they could dive and for how long.

A study was done by the US mil­it­ary in the 1950s regard­ing the forces that could be applied to the human body. Because volun­teers were hard to come by, cada­vers were used to test the forces neces­sary to punc­ture the skin, lacer­ate tis­sues, and ampu­tate or crush vari­ous parts of the body. These tests were also done with pig cada­vers. It was shown that cada­vers don’t behave the same way live people do and that the forces and speeds needed to cre­ate a par­tic­u­lar injury were highly variable.

Since those days, sci­ent­ists and engin­eers have con­tin­ued to study these ques­tions. A key applic­a­tion that is bring­ing this work to the fore­front is col­lab­or­at­ive robot­ics. The indus­tri­al applic­a­tions are many, from using the speed and dis­crim­in­a­tion of robots to aid humans in assembly tasks, to the strength and pre­ci­sion that robots can add to a work task.

Collaborative Robots – early years

In the early 2000s, research­ers were work­ing with ways to lim­it speeds, forces and joint torques to try to pro­tect work­ers. I made a little video mashup to give you an idea of where things were then.

One import­ant area for future devel­op­ment out­side of the industry is in health­care. There are many applic­a­tions where robots can offer humans help, includ­ing sur­gery and patient care. Patient care work­ers often suf­fer back injur­ies from assist­ing patients to get in and out of bed or to the toi­let. There are manu­al hoists that can be used for these pur­poses, but to use them a patient care work­er is needed every time. Patient care work is often very intim­ate, since patients may need sig­ni­fic­ant help to bathe or toi­let properly. 

A col­lab­or­at­ive health­care robot could be used in these cases instead of a human work­er, bring­ing extra speed and strength, and elim­in­at­ing the chance of injury or abuse by or to the patient and the patient care work­er. Here are a few examples of the types of assist­ance I’m think­ing about as I write.

Health­care is cer­tain to be a grow­ing area for these types of per­son­al assist­ance robots.

Today’s applications

Since then, the poten­tial for all kinds of phys­ic­al forms of col­lab­or­at­ive robots has developed. Here’s anoth­er short video mashup of where things are today.

More information needed

All of this work is great, but com­ing back to injury pre­ven­tion is key. The Boston Dynam­ics Spot Mini is great at avoid­ing hit­ting people and objects, but there are many applic­a­tions where robot-to-per­son con­tact is neces­sary. ISO/TC 199 star­ted a pro­ject in 2012 to devel­op a new stand­ard to help guide machine design­ers with respect to the level of force that can be tol­er­ated by people.

Led by Pro­fess­or Yoji Yamada from Nareda Uni­ver­sity in Japan, ISO/TC 199/WG12, Human-machine inter­ac­tions, is start­ing a new devel­op­ment phase for ISO/TR 21260, now titled Safety of machinery — Mech­an­ic­al safety data for phys­ic­al con­tacts between mov­ing machinery or mov­ing parts of machinery and per­sons, formerly titled “Safety of machinery — Mech­an­ic­al safety data for phys­ic­al con­tacts between mov­ing machinery or mov­ing parts of machinery and per­sons.” ISO/TC 199 con­ver­ted ISO 21260 to a Tech­nic­al Report and changed the title at the end of April 2020, due to chal­lenges with achiev­ing con­sensus on the con­tent of the doc­u­ment. Con­ver­sion to a TR will help to get the doc­u­ment pub­lished so that read­ers can start to work with the con­cepts included in the document.

ISO/TC 199/WG12 in Denmark, 2019
ISO/TC 199/WG12 in Den­mark, 2019

ISO/TR 21260 is now enter­ing a new devel­op­ment stage fol­low­ing the end of devel­op­ment as an Inter­na­tion­al Stand­ard. The Tech­nic­al report is planned to be pub­lished by the end of April 2023.

force and injury, Force and injury — How hard is too hard? <span class="caps" data-src=

ISO/TR 21260 Scope

The scope of a Tech­nic­al Report tells you what the doc­u­ment cov­ers, and may also provide spe­cif­ic exclusions. 

1 Scope

This doc­u­ment spe­cifies force and energy related lim­its for phys­ic­al con­tacts between the machine or parts of the machine and people that are caused by move­ment of the machine or parts of the machine as part of its inten­ded use or fore­see­able misuse.

This doc­u­ment cov­ers all types of machines that are designed to func­tion where people can be present and the machine can make phys­ic­al con­tact with those people.

This doc­u­ment deals with con­tact between machines or parts of a machine and people dur­ing their nor­mal inter­ac­tion as part of their func­tion and con­tact not neces­sary for the func­tion­ing of the machine. It encom­passes inter­ac­tions that are inten­tion­al or unintentional.

This doc­u­ment is lim­ited to defin­ing threshold val­ues to avoid harm caused by phys­ic­al contact.

This doc­u­ment does not cov­er effects from con­tact due to

  • burn­ing
  • elec­tric­al shock
  • cut­ting or any risk due to con­tact with a dan­ger­ous tool used or sup­por­ted by the machine
  • addi­tion­al effects due to the con­tact caus­ing a fall

How will this Technical Report help designers?

The focus of the stand­ard is on assist­ing design­ers to apply the first level of the Hier­archy of Con­trols; Inher­ently Safe Design. The stand­ard deals spe­cific­ally with forces applied by the machine, and not to any thermal or elec­tric­al effects, all of which require more study.

The Tech­nic­al Report is expec­ted to be pub­lished by the end of April 2023. You can fol­low the RSS feed for the doc­u­ment by vis­it­ing You’ll see the icon for the RSS feed next to the “Gen­er­al Inform­a­tion” head­ing. You can fol­low the RSS feed using any feed read­er, like Feedly or your web browser.

ISO/TS 15066 – Collaborative Robots

ISO 21260 is one of two doc­u­ments that are destined to hold import­ant places as we devel­op closer phys­ic­al rela­tion­ships with our robot­ic cre­ations. The second doc­u­ment was developed by ISO/TC 299, Robot­ics, and is cur­rently an ISO Tech­nic­al Spe­cific­a­tion (TS). ISO/TS doc­u­ments are used by Tech­nic­al Com­mit­tees to pub­lish inform­a­tion that is believed to be import­ant to the sec­tor served by a TC, but which has not yet developed to the stage where it can be stand­ard­ized. ISO/TS 15066:2016, Robots and robot­ic devices – Col­lab­or­at­ive robots (RSS feed), applies to indus­tri­al robot sys­tems as described in ISO 10218 – 1 (RSS feed) and ISO 10218 – 2 (RSS feed). It does not apply to non-indus­tri­al robots, although the safety prin­ciples presen­ted can be use­ful to oth­er areas of robot­ics. The doc­u­ment spe­cifies safety require­ments for col­lab­or­at­ive indus­tri­al robot sys­tems and the work envir­on­ment and sup­ple­ments the require­ments and guid­ance on col­lab­or­at­ive indus­tri­al robot oper­a­tion giv­en in ISO 10218 – 1 and ISO 10218 – 2.

If you are involved in the devel­op­ment of col­lab­or­at­ive robots, these doc­u­ments are required read­ing. They will also be use­ful to any­one design­ing col­lab­or­at­ive robot­ic applic­a­tions since the force lim­its will be very help­ful dur­ing risk assess­ment of the application. 

If you have any ques­tions regard­ing the doc­u­ments or their devel­op­ment, please feel free to drop me an email or leave a com­ment below.

Series Nav­ig­a­tionUnder­stand­ing the Hier­archy of Controls

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