# Jerk Rate? What the heck is that?

#### Wikipedia: Jerk (physics)

In physics, jerk is the rate of change of acceleration; that is, the time derivative of acceleration, and as such the second derivative of velocity, or the third time derivative of position. According to the result of dimensional analysis of jerk, [length/time3], the SI units for its magnitude are m/s3 (or m⋅s−3); this can also be expressed in standard gravity per second (g/s).

Jerk is a vector, and (as with acceleration) there is no distinct term to denote its scalar magnitude (more precisely, its norm, as, e.g., there is “speed” for the norm of the velocity vector).

In elevator motion control, the jerk rate of acceleration is the difference between smooth efficient operation and uncomfortable “Jerky” operation.

Have you ever been in an elevator that takes off so fast you feel as you need to hold on to something to stabilize yourself or you are going to loose your lunch when the elevator decelerates? Chances are the elevator has a jerk rate that is too high.

Jerk is the initial rate at which a body moves towards full acceleration or deceleration. For the average driver, it’s like punching the gas and going into “Passing Gear” You ultimately are accelerating at a normal speed, but you need that extra kick to get you over the hump.

Have you ever wondered why some elevator doors continue to make a rolling noise after they have stopped opening even though the adjacent elevator has doors operating at the same speed and are smooth? Chances are the jerk rate is too high.

When we look at acceleration in a graph, it is linear, meaning, you can draw a straight line from zero to full speed. To improve performance time, we can increase the rate at which a body can accelerate to an acceptable acceleration speed or wait a little longer to decelerate to full deceleration. In a graph, this would be shown as a curve.

Reasonable jerk rates have been defined by physicist and mechanical engineers alike. Acceptable jerk rates very widely by region. In Asian countries, minimal changes in acceleration and deceleration are preferred. On the US east coast, fast is good, due to the sense of urgency in big business. On the US west coast, quick is desired, but as long as there is consistent smartphone coverage, business happens wherever you are, so the need for a fast jerk rate is not necessary.

The speed of elevator doors is not much different. The laws of physics apply here just as the acceleration and deceleration of an elevator. When it comes to elevator doors, too many consultants spec fast door times to justify their proposed improvements in handling capacity. Unfortunately the overwhelming majority of elevator consultants have no physical experience actually working on elevators or designing elevator components. They look at the maximum speeds allowed by elevator code and that is what they specify. They also rely on sales information provided by the elevator companies.

Have you ever watched a truck commercial on TV. What’s the first thing they show? It’s a truck climbing a boulder pile and going airborne or driving through a muddy rutted road, airborne again, all the while saying Chevy Tough or Built Ford Tuff, or Ram Tough. Do you really expect to be able to use this truck in this manor? If you do, you are a fool.

An elevator should not constantly be operated at it’s maximum regardless of what your consultant says. Yes your elevator company’s salesman or account manager will tell you , of course we can do that. All they are doing is reading the same literature the consultant is reading. The term “Account Manager” and “Consultant” are pretty much interchangeable.

Sure your elevators are capable of doing everything the literature says, for a while. Drive your vehicle at full speed and the decelerate as fast as you safely can, over and over again and see how long your car lasts, not to mention the terrible fuel economy. An elevator utilizing maximum jerk rates or even close to the maximum, will meet specified time requirements for a limited time. What you are not told is your equipment will not last as long as it normally would. Regardless of what your elevator company and consultant tells you, things will break sooner and the down time you experience will be at your detriment not theirs. You have to live with the down time due to premature repairs.

Bottom line and as previously mentioned, physicists and engineers have done all of the heavy lifting and figured out what is comfortable and what is not, for the average human . Elevator manufacturers design their equipment with this in mind. The major companies have tested their equipment at nauseum and agreed on the optimum performance criteria while keeping passenger comfort in mind. That’s how they sell their wares. Before you let a consultant dictate the jerk rate in your spec, check the acceleration, deceleration and ride quality of your elevators prior to implementing any changes to your equipment. If you feel your tenants will accept a more abrupt acceleration and deceleration profile, then you make the decision.