With any mechanical system there needs to be a way to drive the system and Linear Transfer Systems are no different. Linear Transfer Systems are used in a variety of applications such as shuttling tools or moving multi-axis robots as in RTU applications.
Each type of linear transfer system will have its own applications engineering approach to solve the design challenge in the right way. In particular, RTUs or Robot Transfer Unit applications can become quite complex because you need the right mix of performance attributes to get the robot in just the right place at the right time taking into consideration the robot application and the additional loads and inertias caused by what the robot is carrying.
There are many design factors that come into play to produce a linear motion for these applications, including those that are unique to shuttling multi-axis robots that will meet your production and business goals.
Core to meeting your design goals with a linear transfer system or robot transfer system is the drive apparatus. The drive system is at the heart of making the setup perform in a way that will meet your goals. There are multiple factors that go into designing a linear transfer system that in turn play into the selection of a design preference for the drive system and the drive system details.
What are those factors?
Precision - the item being shuttled, perhaps a robot needs to repeatably be placed in the right location within a tolerance that will enable it to perform its function. The range might need to be very precise when a linear transfer unit robot is called upon to load a component in a machine chuck or it may be able to be wider if a robot is fitted with machine vision that enables it to locate a part on a pallet or in a bin. Precision needs to be maintained over a long enough time period between maintenance or calibration for the system to produce ROI.
Speed - a system or robot that’s called upon periodically to move parts between operations may be able to accelerate and decelerate at a relatively leisurely pace as it can be sent on an errand a bit ahead of time to allow for movement. A system handling large castings may need to be speed limited to minimize the loads on the system as it operates. Other systems have heavy demands and light loads and may need to operate quite quickly to achieve production goals for the system.
Durability - systems need to meet overall lifecycle goals for number of operations or number of years for payback of system investment.
Maintenance - “lights out factories” are designed for long operating periods between maintenance or breakdowns. Some tradeoffs may be made for operating performance and maintenance in specific applications.
Envelope - some systems are limited in how long of a vertical reach they can deliver without some design compromises. Longer Linear Transfer Systems narrow the drive system selection down by default.
Other - some drive systems are ruled out immediately by other constraints. A factory without compressed air within reach probably wouldn’t want to consider a pneumatic system because the cost of a compressor room, compressor and air system makes such a choice impractical. Environments which have lots of debris, dust or corrosives may impact the type of optimum system selected.
There are a multitude of drive system options for Linear Transfer Systems. With each method comes costs and pros and cons, and the right selection ultimately resides in the requirements of the system as a whole.
The Types of Drive Systems used for Linear Transfer Systems include:
Driving linear motion, such as an RTU shuttle with a belt, as with anything in a manufacturing process from the conception of conveyor belts, is the first thought. Belts are ideal for a moderate transverse distance. Their ability to effectively and reliably achieve greater distances is limited by the nature of a belt. Since a belt is a tension driven device, the greater the distance traveled, the more the belt sags and as a result a decrease in tension occurs. This loss of tension results in a loss of motion as the belts function becomes limited. This can be rectified by using additional frameworks and linear tracks in a manner that will alleviate some of the sag. A belt is a cheap option for moderate length but belt stretching and therefore maintenance is an issue and consistency accuracy can be a challenge.
Another means to drive a linear shuttle devices such as linear transfer units is with a ball screw. This mechanism removes the sagging, and in turn vibration and springiness of a belt driven system. This type of system is limited in the way the ball screw and its shaft operate. As the shaft deforms under its own weight limitations in length propagate. On longer shafts the primary issue is the screws whip at greater RPMs.
Due to the mild bending of the shaft and resulting screw whip, when the ball screw is rotating at greater speeds the system tends towards its natural frequency. This effect can be minimized using a series of bearing supports, though the length of a single leg of a ball screw between bearing points is still limited. Extensions can be used for longer tracks by adjoining multiple screws to give room for some additional length. There is a significant compromise in length vs design and in linear speed due to the RPM limitations.
Linear motors are a good way to drive linear transfer systems as they allow for relatively precise motion however these are usually used only over relatively short distances due to the cost per foot of these systems. With the use of encoders the motor is able to determine traveled distance to the accuracy of the encoder and thereby control the location of the linear transfer shuttle. Due to their inability to operate at the lengths that most production manufacturing operations require, they are often seen for laboratory functions. Their ability to provide great mechanical accuracy comes at a cost of load capabilities, length and cost.
Pneumatic driven systems are a great option for linear transfer system applications with only two points of use, typically at either end of the track or between two stop points. This is a low cost solution that can quickly transverse the required distance at quite high speeds. This is a system that would only be used when there are two stop points as it has limited accuracy and would be difficult to control at various points along the axis.
Pneumatics come with their own drawbacks. You need sufficient air pressure and supply to feed the system while meeting your other air supply needs. Compressed air is expensive energy-wise with significant energy losses throughout the system. Pneumatic equipment can be noisy and leaks are a source of maintenance costs. For rapid shuttling operations between two points where you have adequate infrastructure and noise isn’t an issue, pneumatic linear transfer system drives can make sense.
A hydraulic driven system works similar to that of a master cylinder and brake cylinder in an automobile. Pressure is built and exerted onto the linear transfer driven system which results in translation down an axis. This method tends to have limited accuracy depending on the method of which the pressure is applied. With greater expense at the application of the pressure the accuracy of the system will increase. This system is considered high maintenance and messy as it will tend to partially leak, as with any hydraulic system. For very high loads and short distances needing low precision this can be an effective option.
High-Precision Helical Rack and Pinion Sets:
The most common means to drive a linear transfer system is with the use of a rack and pinion set. They are capable of handling good distance and have a great level of accuracy in the applications of servomotors and gearboxes to accurately determine the total rotation of the pinion along the rack. The pinion and its drivers would be attached to the slide or shuttle plate driving the moving unit directly and the drive moves with the driven robot in an RTU application.
The control of the unit is directly integrated and coupled with the axis in the robot controller which eliminates the requirement for any additional controls to control your linear automation motion. Backlash in a gear reducer can be minimized by using cam systems with low backlash gear heads to achieve high accuracy and repeatability on all systems. Racks can be extended nearly indefinitely to adapt to very long linear motion needs as long as the umbilical supplying the shuttling unit is adequate to the length.
LazerArc finds high-precision rack and pinion sets to be the ideal balance of cost, precision, application flexibility, cost and reliability to cover most linear transfer system needs.
Want to learn more about how to select the right system? Pick up our guide below!