Chapter 10 ‐ Transport belts part 2 ‐ Underground belts and splitters

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Transport belt tiers and capacities

A regular transport belt unit can carry about 7.5 items across each lane every second, giving it an overall throughput of 15 items per second. This is a good capacity that can support several machines. For example, given that an electric mining drill outputs 0.5 iron ore units per second, a regular transport belt can support up to 30 mining drills working nonstop while feeding up to 24 iron smelting furnaces nonstop. This should also give an idea of the scales at which mass production can work in Factorio.

If you were to use only a single regular transport belt for every application, you would be able to finish the game this way, although slowly. Some jobs can benefit from using a fast transport belt instead, which has double the speed and thus double the throughput rate as a regular transport belt. Alternatively, you can use two regular belts running in parallel, and maybe even keep them balanced by passing them through the same belt splitter machine. The disadvantages of using higher tiers of transport belts are that they are significantly more expensive in terms of iron gear wheels needed, and that they are too fast for burner inserters to pick up from.

To summarize all variants: A regular transport belt (yellow) can move 15 items per second, a fast transport belt (red) can move 30, and an express transport belt (blue) can move 45.

Underground belts

Underground belt units are items that allow you to build straight belt segments underground. The advantage of having belts go underground is that they can pass under obstacles without colliding with them, and multiple underground belt segments can overlap without having any issues.

The item that is named the underground belt is actually a 1 by 1 underground belt entrance chute with its front face having a belt going into it and its back face disappearing into the ground. The chute can be rotated face any of 4 directions before being placed. Rotating it after placing it flips its travel direction to make it an exit chute instead, with its front face having a belt moving outside instead of inside.

Underground segments are formed automatically and for free when you place an entrance chute and an exit chute in a valid configuration. The exit chute must be rotated to face the opposite direction as the belt it is part of, and it flips itself for you when placed correctly. As of version 0.4, the building preview for an underground belt unit explains where it can connect, if any. For tier one (yellow) underground belts, the underground segment can be up to four tiles long before becoming invalid. For higher tiers of underground belt units, this distance is longer. Note that the entrance and exit units for an underground belt must be the same tier for them to interface.

The surface tiles above an underground belt segment are free for building on, and they are often used to run perpendicular belts that would otherwise collide with the belt segment that is now underground. The underground segments can also pass under cliffs, ore patches, and water bodies without a problem.

Splitters

Splitters are belt-based, multi-purpose machines that allow creating complex belt systems with high throughputs and extensive lane management. Splitters are two tiles wide and can face four directions, with the back side having two belts entering (inputs) and the front side having two belts exiting (outputs). Their functions include the splitting, merging, balancing, prioritizing, and filtering of belts. In terms of splitting and merging and balancing, the current function of a splitter is automatically defined by which inputs and outputs are being used, without any additional configuration being needed. Unused inputs do nothing and unused outputs simply hold a few items like the end unit of a regular belt segment. Meanwhile, filtering and prioritizing functions can be used by changing the splitter settings.

And important feature of splitters is that they preserve lanes across their inputs and outputs. For example, if the input belt has items only on its right hand lane, the output belts will both have items only on the right hand lane.

Splitter splitting

The primary usage for a splitter is evenly splitting the contents of one or two input belts onto two output belts. This way, items flowing in from one source can be sent to multiple destinations. If the two input belts have different items, the two outputs usually get a mixture of both, while respecting the original input lanes. If you set one of the outputs as the priority, the splitter will direct all items to it until it is full, which makes remaining items go to the other output. If both inputs and both outputs are being used, the function of the splitter is better described as balancing.

Splitter merging

If a splitter has two input belts and one output belt, it will evenly merge the contents of the two belts, while respecting the original lanes. If a lane on the output belt is full, items moving on corresponding lane for both the input belts will stop. If you set one of the inputs as the priority, the splitter will collect all the items it can from there and it will take from the other input belt when it has space left over.

An alternative to splitter merging is building sideloading belt junctions, which are simpler but they force all contents of one belt on the nearer lane of the other belt. This halves the overall throughput and mixes lanes contents.

Splitter balancing

A common use for splitters is balancing the item flow on two belts that are reserved for carrying the same item type. The splitter is placed on a point where the two belts are running side-by-side. From the splitter’s perspective, there are two inputs and two outputs. By taking the items equally, the splitter ensures that both inputs are being used. By distributing output items equally the splitter ensures that both outputs are being fed. Hence the belts become balanced.

Balancing is necessary for the even distribution of a limited supply input resources to multiple high-demand outputs. On the other hand, it is not a problem if the output demands are low such that the output belts are backing up.

Balancing with splitters is not limited to only two belts. Many in the Factorio community have been developing fancy multi-splitter combinations that allow perfectly balancing all sorts of transport belt combinations. However, perfectly even distribution is not a requirement for balancers, and this question only becomes relevant when belts have very high traffic. If you want to balance three or more belts that are running side by side in a basic way, it is enough to place one splitter to connect every neighboring pair, and then the overflow from underused outputs will do the rest of the balancing for you.

Lane balancing using a splitter and a merge junction

Generally, the lanes of a belt that is reserved for single item are unbalanced due to the unequal placements of input and output inserters. This is not much of a problem since an inserter can always take from the other lane if the nearer lane is empty. However, maximizing the utilization of both lanes may be desirable for a belt with very high traffic on it such that one completely full lane is not enough capacity. This can be done using a splitter and a safe merge junction made from belts.

Given that a splitter does not mix any of the left and right lanes for any of its belts, it will not balance them either. However, this can be achieved by combining it with a safe merge junction. To lay out how this works, first we use the splitter to divide one unbalanced input belt into two unbalanced output belts that both have exactly half of the total item count of the input belt. Then, we make both output belts sideload onto a new belt at the safe merge junction. Since the sideloading pours two lanes into one, each lane of the new belt now has exactly half of the total item count of the input belt. Therefore our final output belt has its lanes balanced.

Belt compression using a splitter and a merge junction

A fully compressed belt is a belt that has no gaps on it. Every stopped belt is compressed since the items on it have nowhere to go and they fill up the gaps in between them. For a flowing belt, compression is possible by squeezing the traffic of two belts into a new third belt.

We do this by feeding the two belts into a splitter and creating a merge junction with the splitter's two output belts. The result of this assures that half of the total input from the two input belts ends up on one output lane and vice versa. If the two input belts have enough traffic in total to sustain one output belt, this setup will ensure that the output belt is fully compressed. Meanwhile, given that we are putting two belts into one, this reduces the overall throughput of the system unless the output belt is a faster belt.

Setting splitter priorities

You can set the input priority of a splitter by selecting it and pressing SHIFT + LEFT ARROW or RIGHT ARROW, depending on the direction you want. Pressing the same direction twice will restore the default of equal distribution. Similarly, you can set output priority using CONTROL + LEFT ARROW or RIGHT ARROW.

To recap, the input priority determines which side the splitter will collect from first, and it will collect from the other side only if it has space left over. Similarly, the output priority determines which side the splitter will deliver to first, and it deliver to the other side only if it completely fills up the priority side.

Splitter filtering

Splitter filtering is a different function to all the others because it is about separating out a specific item type, rather than distributing all item types. A splitter can be set up to reserve one of its two outputs to a specific item, while all other items are directed to the other output. This is very useful for sorting out belts that have various items on them or belts that have their lanes get mixed up.

You can set the item filter by holding in hand the item you want to filter out and pressing CONTROL + LEFT BRACKET om the splitter. You can then select the filtered item output direction using CONTROL + LEFT ARROW or RIGHT ARROW.

Note that for a filtering splitter, if one output is full, then the other output will be blocked too because of items having nowhere to go. This is in contrast with other splitter operations, where one output backing up does not cause a problem.

Transport belt questions and answers

What options are there for putting items on or off belts?

Almost all types of adding or removing items from a belt can only be done with inserters. Here are the exceptions:

• Mining drills can drop their outputs onto a belt directly.
• The engineer can pick up items from belts directly (hold the "F" key) or place items onto a belt directly (hold the "Z" key over the belt with the item stack in hand).
• Several mods including some compatible mods for Factorio Access allow using loaders, which are machines that make belts directly flow into buildings or vehicles.

What is belt traffic?

This refers to how much transport capacity is being demanded from the belt. It is determined by overall amount of items being removed from the belt. It does NOT refer to how full the belt is right now.

What options are there for combining two belts?

You have many options:

• You can make one belt sideload onto the other belt although this halves the throughput of the belt being poured, and you are forcing 3 lanes in total into 1 lane.
• You can pour the two belts into a safe merge junction and create a third belt. This ensures that the contents of the two belts do not mix with each other. This forces the two lanes of each input belt into one lane of the output belt, and can reduce the overall throughput unless the third belt is a faster belt type.
• You can input the two belts into a splitter machine. If you take only one output from the splitter, then the contents of the input belts will be combined. However note that the lanes will be preserved: The left side input lanes will add up in the left side output lane and vice versa.

What options are there for splitting one belt into two or more belts?

You have 2 options for this:

• The best way to do this is to feed the belt into a splitter machine and to take two output belts from it. The output belts can head off in opposite directions or one of the output belts can continue in the direction of the input belt. Adding more splitters to the output belts will give you further output belts as needed.
• You can also use inserters to take some of the items from one belt and load place onto a new belt. It is likely for the inserter to be either too fast or too slow for this job and the two belts will probably not be balanced.

Can belt lanes be simply ignored?

If you keep only one type of item on a belt and if the belt has low traffic, then the lanes can be ignored entirely. When there is high traffic, you may want to review your sideloading junctions so that both lanes of the belt is being used.

Is it better if a belt is kept full or empty?

A belt that is full overall means that it has more input overall than output overall. This is generally a good target to aim for when designing a factory so that empty belts can be used as a way to alert you that an input stream has been disrupted. Another advantage of aiming for full belts is that it allows for splitters along the belt to reroute excess items from one full output to another output if the full output is not being consumed at the moment.

How do I make two belts pass over each other without mixing them?

You need to move one or both of the overlapping belt segments underground.

Next page: "Chapter 11 - Assembling machines and automated production"